Here's a really easy one to do, and that's great for gatherings of 3 or 4 people:

Bong to Hookah Adapter

1. Assume you have a bong. Any bong will do, as long as it can stand up on its own without your holding it.

2. Tape over the carb hole, if applicable. Leave the bowl and slide where they are.

3. Take a 20 Oz.bottle of any kind; all that matters is that it tapers somewhat. Turn it upside down, and place it into the top of your bong. Mark where the bong touches the bottle around the rim of the bong. Remove the bottle and cut off the top about 1/2" toward the top from your line, such that there is about 1/2" overlap between the bottle and bong when in position. This will facilitate better airflow than leaving the bottle intact. Once the bottle is properly placed, tape it down to the bong, making sure there are no air leaks.

4. Melt/ cut holes around the rim of the bottle, and insert tubes-- the number can vary, but just consider that you'll have to stop up any unused hoses when using this things, or else you'll draw air and not smoke. Seal the hoses in place with tape/wax/silly putty.

5. Add water, insert bowl, pack weed, light. Get all your friends (1 per hose) to suck. You'll fill the chamber, and then Voila! Your once single-user bong becomes a stylish--if wonderfully ghetto-- hookah!