Step 1: Real hardware... or emulation?
First of all, if you've got your hands on real original 80's or 90's Macintosh hardware and want to try on it, then you diserve a handful of internets my friend! That's awesome! The challenge you will face is how to input files you get from the internet into the old Mac. To do this, you will need a floppy drive and MacDisk (or Basilisk II) to help you copy files on a floppy disk that you will insert into the old Mac.
Out of luck for a real Mac and only got emulation to try? It's OK, that's how most people do nowadays anyway. Here is the best selection of emulators that will allow you to run every single classic Mac OS version ever released:
SheepShaver, a PowerPC emulator runs Mac OS 9.0.4 down to Mac OS 7.5.2
└─── BEST FOR: 1995 to 2001-ish PPC software
Basilisk II, a color 68K emulator, runs Mac OS 8.1 down to Mac OS 7.0 (can read/write on 1.4MB floppy disks!)
└─── BEST FOR: 90's 68K software and making backups of your old floppy disks before it's too late!
Mini vMac, a B&W Mac Plus (68k) emulator, runs Mac OS 7.5.5 down to the original Mac OS 1.0 from 1984
└─── BEST FOR: 80's B&W software that typically runs on a 9 inch screen
QEMU with PPC OpenBIOS, an emulator capable of running Mac OS 9.1, 9.2 and early Mac OS X (10.0 to 10.5)
└─── EXPERIMENTAL / summer 2016: no sound, very slow, but boots Mac OS 9.2.2 and early Mac OS X versions
So, depending on which version of Mac OS (and/or CPU) you'd like to emulate, you need to choose the above emulator accordingly.
Step 2: Basic System Tools
STUFFIT EXPANDER... What most people get confused with is how old Macintosh files were stored. Unlike any other platform (Windows, Mac OS X, etc...) the files on earlier Mac OS versions contained a resource fork that *COULD NOT* be stored/transfered on anything else but a Macintosh file system, except when compressed with Stuffit or BinHex. This means that you CANNOT extract a .sit archive containing Mac files under Windows, ever. Sure, if you try it, you'll see files extracted, but they will all be corrupted once you move them to Mac OS, since they will all lack the resource fork. Now, I see you coming: "Then why is Stufit Expander published for Windows too?" Well, Stuffit Expander supports lots of compression formats, including ZIP, but it's certainly not for expanding Mac .sit or .hqx files, that's for sure. Every file you see on the internet for download to an old Mac OS ends with .sit or .hqx. To uncompress those "stuffed" archive files, first you need a Mac OS environment and then: if you run Mac OS 7.5 or newer, you need Stuffit Expander 5.5. For anything earlier than Mac OS 7.5 you can grab a copy of Stuffit Expander 3.5.1 but note that it does not extract some of the archives made with Stuffit 4.x or newer. The file compression standard in the 80's was Mac Binary anyway, which Stuffit Expander can extract among other formats. Stuffit Expander needs to be on your system before attempting to copy files from any source, so here's Catch-22: When you set up an old Mac OS that does not come with Stuffit Expander, how can you transfer Stuffit Expander itself from a PC computer without destroying its resource fork since you can't uncompress Stuffit Expander without Stuffit Expander, right? That's the only situation where you need to use the Basilisk II emulator, which has integrated floppy drive support, so you can uncompress Stuffit Expander and copy it, complete with its resource fork onto the floppy disk, which should open fine on the old Mac computer afterwards and allow you to transfer compressed files from now on using traditional ways. Any .sit or .hqx file can be transfered on any file system, Mac formatted or not, without getting corrupted.
DISKCOPY... You will find other types of archives that we call disk images (files ending with .img). DiskCopy is Apple's tool that shipped with classic Mac OS version 6.x up to Mac OS 9.2.2 that allowed for a perfect replication of a disk or a folder with everything in it, keeping the window and icon layout/positions, comments, color labels you might have applied, creation/modification dates, etc... it looks good, except that as previously stated, since they're not .sit or .hqx files, they *CANNOT* be moved/stored on a non-Macintosh file system, since this would render the disk image corrupted. Always Stuff (.sit) or BinHex (.hqx) all your files before moving/storing them outside of the classic Mac OS, disk images created with DiskCopy included. There is a way to make classic Mac OS disk images binary safe (for example if the image is created with special software in Windows or Linux, resulting in a slightly different format, like .dsk file instead of .img) but the ones created by DiskCopy are not.
TOAST... it creates (backup), mounts and even burns CD-ROM disk images like .iso and .bin. It can also copy CD-ROM's to CD-R directly. You will need Toast if you plan on downloading CD-ROM .iso disk images or if you want to backup/archive your own CD-ROM's before it's too late. If you're using Mac OS 7.x you can grab a copy of Toast Deluxe 4 and if you're running 8.5 or 9.x then get a copy of Toast 5 Titanium instead.
Virtual DVD-ROM/CD Utility... it's the absolute best, simplest and most efficient disk image mounting tool ever made. It's incredibly tiny file size (42KB!! Heck, the read me file is larger!) deceptively hides ultra efficient functionality that allows a user to instantly MOUNT ANY DISK IMAGE FORMAT and it doesn't even care about the type/creator code at all. Some software that required the CD to be put in the drive back then won't install or run without their ISO disk image mounted and in some cases where images mounted with Toast would not work, it works with this app. Virtual DVD-ROM/CD Utility can even save an ISO disk image of any of your drive to the push of a button. No option to set, just pure instant gratification. It runs on Mac OS 7, 8 and 9 without any issue! Better yet, it's public domain software so it's free. This app should be installed on every classic Mac OS hard drive: Virtual DVD-ROM/CD Utility.
MACDISK (for Windows)... if you want to move files from a PC to an old Mac OS (or the opposite) using floppy disks, then you will need a tool called MacDisk. This program allows Windows OS to format, read and write files on Macintosh formated floppy disks (also supports CD-ROM, USB drives, Iomega ZIP drives, etc...) but *NOT* on the older 800KB or 400KB floppies, it only works with regular 1.4MB floppy disks. Keep in mind while using this program that if you copy old Mac files on a PC file system, it will instantly lose the resource fork and this will permanently corrupt them, unless you compressed them as .sit or .hqx under Mac OS beforehand. MacDisk is useful in the scenario where you already compressed (.sit) your files and you want them copied onto a Macintosh formated disk. Once you insert the disk on the old Mac, then using Stuffit Expander you will be able to extract and use the files. Note that if you use Basilisk II as an emulator, it already has integrated Mac floppy disk support, effectively using your PC floppy drive so you do not need MacDisk, you could just insert your floppy disk in the drive and Basilisk II will mount it in the Mac OS so you can directly use it.
Step 3: Mounting disk images
One of the problems retro Mac enthousiasts face nowadays is figuring out how to mount random disk images in order to access the game or app in it. First of all, you have to know that there were at least a dozen disk image formats back then (altough only a handful of them were widely used) and fortunately, there is a very nice disk image mounting tool for classic Mac called Virtual DVD-ROM/CD Utility that mounts just about all of them. Other than that, each disk image format is tied to one app (or in some cases a couple apps) that can mount them. Also, just because you could once mount a .img file with DiskCopy does not mean that you can mount all of them with it nor with the same version of DiskCopy. Two very important notes about DiskCopy: First, altough DiskCopy will successfully mount 400KB (MFS formated) single sided floppy disk images under Mac OS 7 and older, it will NOT mount them under Mac OS 8.1 to 9.2.2, so if you've got a very old 400KB disk image, the only option is to mount it under Mac OS 7 or older. Second, disk image files created with DiskCopy *CANNOT* be copied/transfered to a non-Macintosh partition. They will get damaged and lose their resource fork, rendering them useless. So, do not unzip or expand DiskCopy images on Windows or Linux. Expand them under Mac OS to be safe. With all of that said, the rule of thumb is: DiskCopy is good to mount small (e.g. floppy or zip) disk images (often ending with .dsk or .img) that you found stuffed (.sit) or encoded (.hqx) on the internet and for all Apple disk images such as software updates and installers. Tough, if you run across a .img file that DiskCopy does not recognize, then it's probably a ShrinkWrap disk image. ShrinkWrap was Aladdin's (the author of Stuffit) proprietary format that DiskCopy could not mount, so make sure you keep a copy of ShrinkWrap handy to mount some of the 90's floppy disk images. DiskCopy and ShrinkWrap were pretty much the standard in disk imaging from mid 80's to mid 90's, except for some fancy users who made disk images using DiskDoubler so if you have a 90's disk image about the size of a floppy or less and it doesn't mount with aforementioned tools, then try DiskDoubler. Then around the mid 90's, Toast came into play with CD imaging. So if your image is big (e.g. 100MB or more) or ends with .iso, .cdr or if it did not come stuffed (.sit) or encoded (.hqx) then it most probably will mount using Toast.
Looking for a "fast" (10/100/1000 mbps) PCI network card for your old PowerMac that works on both Mac OS 9 and Mac OS X ? D-Link DGE-528T is one of the rare universally compatible fast PCI network cards reported to work both on Mac OS 9.2 and Mac OS X 10.2.
Transfering files between 2 old Macs via serial connection
Using AppleTalk, very old Macs could be hooked up together via the printer port. Read the instructions on this Classic Mac sharing guide.
Archiving 1980's Mac floppy disks
The first 1984 and 1985 Macintosh models featured a 400KB (known as: Single-Sided "SS DD") floppy drive. Note that most of the 400KB SS DD floppy disks you will find are formated using a long deprecated and unsupported MFS partition format. MFS partitions cannot be mounted at all using anything above Mac OS 8.0 (not 8.1) and only up to Mac OS 7.6 can write files to it. Macs from 1986 and 1987 featured a 800KB (known as: Double-Sided "DS DD") floppy drive. From this point on, the partition format was standard HFS which, 30 years later, can still be mounted and written to using Mac OS X, to the opposite of the unsupported MFS partition format. The final change was in 1988 when Apple released the first SuperDrive equipped Macs, having a fully standard 1.4MB floppy drive, while still reading both the 400KB SS DD and 800KB DS DD floppy disks.
Now, if you do NOT have an old Mac and only have a PC to work with, please note that both low density floppy disks (400KB SS DD and 800KB DS DD) CANNOT be read with the PC's floppy drive. Apple had different firmware/hardware in their drives allowing for slightly more DATA to be saved onto the disks and as such, PC floppy drives are totally incompatible, no matter the year or brand. With that said, thanks to an indie add-on controller board, it is possible to extract the contents of those floppies and make a disk image out of them. The board is called KryoFlux. It connects between the motherboard and the floppy drive and when paired with their software, can effectively save disk images of the floppies that you could use in emulators, such as Mini vMac. For 1.4MB floppies, it couldn't be easier: Just launch Basilisk II and insert your floppy disk! It will mount on the emulated desktop and allow you to read/write (which at this point, you should make a disk image using DiskCopy).