Follow these steps and you'll be golden
Find an empty 2 liter.
Find some tin foil and rip of a 3inch by 3inch piece.
Place the tin foil over the top of the 2 liter and let it sink in about an inch.
Tape the edges of the tin foil around the 2 liter.
Poke small holes in the tin foil. (these is where you'll place the weed).
Poke a hole in the side of your 2 liter.
Poke one more hole in the side and jam a straw in it. Whola! You have a home made bong.
Make sense?