When I first learned about this device, it was called a "Pinhole Microscope." I don't think it has enough magnification to be considered a microscope, so I'm just calling it a magnifier.
Clear beverage bottle (a half-liter water bottle in this case)
Dark, opaque plastic piece, large enough to cover the top ot the bottle. (top from a peanut can) Cardboard, like from a cracker box, will also work.
Knife, scissors, or both (Okay, I'm using tin snips. They were easier to find!)
CA cement (Superglue, Crazy Glue or the equivalent)
Pin (not shown)
Sandpaper (not shown)
Step 1: Cut the bottle
Cut the neck off the bottle just below where it starts to widen. This should give you a section about 1" (25mm) long. Sand the cut edge smooth, so nobody cuts themselves on it.
Step 2: Cut the disk
Cut a circular piece of your opaque material, just large enough to cover the top of the bottle.
Step 3: Glue
Glue the material over the top of the bottle. You may want to roughen the bottle top a bit with the sandpaper first.
Once the glue is dry, poke your pinhole in the center of the opaque material. Start small, it's a lot easier to make the hole larger than it is to try to make it smaller.
Step 4: Finished
Put the object you wish to view under the magnifier, make sure you've got plenty of light, and look through the pinhole!