My gravity bong is slightly different than flamingweasel's, so I'll describe mine here

Ingredients/Supplies:
  • One 600 ml pop bottle. (larger if you like, I suppose, this is big enough for me)
  • Bowl assembly of some sort (I use one I bought from a local head shop)
  • Knife, or any suitably sharp object
Procedure:
  • Cut a small hole in the bottom of the bottle
  • Cut a hole in the cap to put your bowl in
  • Put your bowl in the cap-hole
  • Fill bowl with your favorite cumbustable, plug the bottom-hole with your finger, fill the bottle with water, put the cap/bowl on, light, and release the water.
  • enjoy
According to Ground Control's definition there, I am a Certified Bong Technician.

Note:I've tried gravity bongs of the sort described by donfreenut, and they are highly (no pun intended) recommended.