Naphthalene charges are fun and easy to make. Their safety, ease of use, and easy to obtain components make them a great beginning point for the aspiring amateur pyrotechnicican. I have heard that these charges are what some special effects artists use in movies to produce those neat mushroom shaped fireballs with black edges. Though this is a safe project, remember to wear proper safety gear and use proper precautions, it is important to do this habitually. CAUTION: This stuff stinks horribly of old people. Don't do this inside unless you living in a retirement community.
Naphthalene mothballs - Make sure to get the naphthalene kind, no substitutes will work. Enoz old fashion moth balls work well. Moth flakes are even better to obtain, because they are easy to pulverize. Here is a link where you can purchase these: walgreens.com.
Greenmix/BP/Pyrodex - You will need a flammable powder to jumpstart the ignition. I use greenmix, but I understand that BP and Pyrodex work fine. I have heard that smokeless powders will not work.
A container - In order to get a neat effect, you will need something fairly large. I have had good success with a small cardboard box.
Cab-O-Sil - This is not needed, but I recommend using it the get the naphthalene to a nice fluffy powder.
Coffee Grinder/Mortal & Pestil/Hammer - You'll need something to grind up the moth balls, a hammer and plastic bag is an easy primitive solution that will work fine.
1) Crush up the mothballs into a fine powder. I add a teaspoon full of cab-o-sil to every small handful to keep everything from clumping together. Scott from rec.pyrotechnics described a process with a sledgehammer and a pouch made out of blue jeans. Sounds like a great way to quickly pulverize some nap.
<-- Turn them from this, into this --> 
2) Drill a hole at the bottom of your container for your ignition device. Insert your ignition device (I recommend visco),
3) Put a layer of your flammable powder in the bottom of the container covering the fuse. You will want a %15 layer of this making sure that it covers the fuse.
4) Now you will need to insert a layer of %30 flammable powder, %70 naphthalene (by weight). You can experiment with how much to use, %20 should be adequate.
5) Now fill the rest of the container with naphthalene, being sure to leave a good amount of open space at the top. I usually fill my containers half full so that the naphthalene doesn't blow out unlit.
This is a popular drawing of a nap bomb (not my work). You don't need an endplug or the toilet paper wadding. There should be about %50 of the container empty at the top of the charge, and you don't need as much BP/pyrodex as pictured in the diagram.
These are good 99.5% naphthalene mothballs.
A video of the effect - I used around 10oz of naphthalene for this one, and a couple of oz of greenmix to get it started.
Below are pictures of the box used in the video. You can see it leaves quite a bit of mess behind.
