Light a cig, burn a hole in the side of a two liter with the cherry (about halfway up), slide an emptied out pen in there (actually, you'll need a couple of them taped/glued together somehow), and attach some sort of metal bowl (not aluminum) to the stem. Then burn another hole a little above where you slid the stem in if you want a carb. The reason I tell you to burn the holes with a cigarette is because when you do, the pen is a *perfect* fit, and will usually be almost completely airtight.

That's basically your traditional 2-liter bong.