You will need:

An old mac, with the original form factor (SE, classic, classic II, 512k, etc). Note that 
every particular model has a slightly different internal layout. This tutorial uses a Mac Classic.
Cost: free from dumpster diving, try school surplus if you don't want to dig in the trash.

A fish tank that will fit into the computer case. I found that the "Penn Plax New World 
Explorer" (Available at Fred Meyer) fits almost perfectly with some small modifications. 
Cost: $5.45 from Fred Meyer

A star screwdriver with a long shaft. Multiple extenders and a universal 
screwdriver set with star bits are much cheaper than buying Apple's specific tool. 
Cost: $5-$10 depending on brand and quality

A small saw, such as a hacksaw or electric saw. Pocketknives work but are slow.
Cost: depends on specific tool

Gravel, fish, and any personal accessories. An air bubbler and lights are optional. 
Cost: $1-5 for bubbler from thrift store, $0.10 for goldfish, free lights (salvaged)

Step 1:

Assemble your tools, remove the four screws from the rear of the case (the two inside 
the handle well will require the long screwdriver, Apple really didn't want you opening these). 

Step 2:

Remove the case and take out the guts, save anything that looks usefully. I used the motherboard 
for a backdrop on the fish tank. Leave the metal motherboard / drive mounting bracket in place, 
since it provides a good platform for the tank. 

Step 3:

The top of the macintosh must be removed to allow the tank to fit in, measure it and cut a hole large 
enough to access the tank and to support the rear of the tank against what's left of the mac's handle. 
Try to do a better job than I did here, my cut ended up being a little sloppy since I had a cheap saw.
I had glued the motherboard to the rear of the tank at this point, so the hole had to be cut slightly 
larger to accommodate it. Install the bubbler unit (if you have one) somewhere inside where it won't vibrate. 

Step 4:

Fit the fish tank into the computer, The Penn Plax one fits almost perfectly, but  must be 
trimmed of the plastic flange at the top for it to fit into the rear part of the case, do this with 
a razor or sharp pocketknife. A tight fit will eliminate the need for gluing the tank in place. 

Step 5:

This one is optional, Install any accessories and run electrical wires through one of the holes in the rear 
of the computer case. Tie the cords around the metal frame first so that pulling on them will not damage 
anything. Leave enough outside the case to reach electrical outlets or extension cords. I spliced the wires 
for my LED light unit (salvaged from an exit sign found at the dump) into the power cord for the air 
pump, this isn't recommended unless you have experience with electrical repair and safety procedures. 

Step 6:

Final assembly, put the tank in place and slide the two halves of the case together. Ensure that
everything fits correctly, that nothing is forced or jammed, and that it looks the way you want it.
You can glue the two parts of the macintosh case together if you want, but you might need to get to the
bubbler or lights sometime in the future (or install some if you don't have them already). 

Step 7:

Clean up your work area (cutting the case leaves lots of plastic debris). 
Fill the tank with water, turn on the power, install fish, and sit back and relax. 
Goldfish don't crash as often as Windows, and are more relaxing to look at.