I've seen this all over the internet, and decided I would make one myself. There isn't a whole lot of variation that can be done to the design, and this is pretty much a direct copy of the stove design found here: http://www.backpacking.net/makegear/altoids-stove/index.html
This is a cool idea, very cheap to make, and I think it needs more coverage, so here it goes, my first instructable, how to make an Altoids tin camping stove!

Step 1: What You Need:

Materials:
A mint tin, for this one we're using a rectangular Altoids tin.

A section of fine metal mesh, windowscreen works great.

Some perlite, which is cheap, and easily found at most garden supply stores. I've heard you can substitute fiberglass insulation for the perlite, but I have not tried this and cannot confirm.

Tools:
Some shears or other tools to cut the metal. Heavy-duty scissors should work fine.

A ruler to measure the tin and screen.

A permanent marker to mark your measurements.

Optional:
Some sandpaper to remove the paint from the tin.

Step 2: Measuring the Tin and Screen

Before you cut the windowscreen, you need to measure the dimensions of your tin. Obviously, a square tin will be easier to cut for. The Altoids tin we're using happens to measure a little under 9.5 cm x 6 cm, but if you cut the screen at those dimensions it should fit. Mark your measurements with a permanent marker on the windscreen.

Step 3: Cutting the Screen

Use your metal cutting instrument to cut along the lines you just marked. You will need to trim the corners slightly to fit the rounded corners of the tin. I trimmed the corners 1 cm diagonally. Trim the rest of the screen accordingly to fit.

Step 4: Filling the Tin

Fill the Altoids tin with perlite, as close to level with the top as you can. Try to get the largest chunks of perlite.

Step 5: Fitting on the Screen

Now, push the screen on to the stove. Fit the corners under the rolled edges of the tin, and once you get it in it should be in there for good. Once that's in, you're ready to go!

Step 6: Tips for Use, and How It Works

Use ONLY Denatured Alcohol, 91%. 70% is also available but has too much water and will smoke. You can find the fuel at a hardware store, and an example is pictured below.

Copied from Paul Dryer's site: http://www.backpacking.net/makegear/altoids-stove/index.html

"I use a little plastic spring water bottle as my fuel bottle and find 3 capfuls burns plenty long for most meals. Also for shorter trips, a 4 oz. nose spray bottle with the internal straw removed makes a good fuel bottle. You can squirt fuel (Alcohol Only !!) right into the stove....un-lit, of course! You can even fill the stove to the brim and be ok. To simmer simply cover part of the screen with a bit of aluminum foil. Boil times are about the same as a soda can stove.

Principle of operation: Alcohol 'wicks' up into the perlite for initial lighting. As the perlite/stove heats, alcohol reaches vapor pressure and accelerates through the screen to the flame. Looks like an esbit tab on steroids when burning and if used in an esbit stove, the height is perfect."

Couldn't have said it better myself. Additionally, I've found that keeping the lid attached is a great idea, because you can essentially "turn off" the stove by flipping the lid closed!

WARNING: The flame is INVISIBLE, so be careful! Be safe and enjoy!