Use your sharp knife to cut the bottom off of the bottle (or the smaller bottle if you’re doing this with two bottles). The more of the bottle that you leave, the more smoke you’ll be able to generate, the more you’ll be able to smoke.
For people using two bottles:
Cut the top off of your bigger bottle, ensuring that your smaller bottle will be able to fit into what remains, and fill the bottom with water.
For those using a bucket: Fill your bucket, or whatever you choose to use, with water.
Take the bottlecap off of your smaller bottle and cut a hole in it with your sharp knife. It must be big enough to be able to suck air through, but not too big, otherwise your aluminium foil and weed will fall through.
Shape your Foil
Shape your fooooil braaaah, said someone cool at some point. Anyhow, you have to wrap your bottlecap in foil, shaping it in such a way that there is a little divot in the middle, over the hole you made in step 3. You then have to poke a couple of holes through the foil, for ventilation. Make sure not to tear the foil!
You can also use a bong piece and stick it through the hole if you have one handy. This is honestly a more convenient option if you do have one.
Load your Bowl
Now the fun begins. Fill the divot you created with some bud and put it on the smaller bottle. Don’t screw the bottle closed yet though.
Submerge your Smaller Bottle
This is probably the trickiest, and therefore most stressful, step of smoking a gravity bong. Submerge your smaller bottle (the one with the weed stacked at the top) into your bigger bottle/bucket, until just the cap is left above the water. Do this slowly and carefully, as many people (myself included) have fallen afoul of doing it too quickly and losing all their weed.
Screw the bottlecap closed once you have successfully submerged the bottle.
Light it up!
Light up your bud and slowly lift the bottle up out of the water. You should notice that the bottle fills with smoke, but if it isn’t, the probable reason is that you haven’t screwed the bottlecap on properly. Make sure to stop lifting the bottle just before it is fully out of the water, otherwise you will lose all of your smoke.
Unscrew the Bottlecap
The next step is to unscrew the bottlecap and put your mouth over the opening.
Push the bottle back into the water
Get ready, because you’ve finally arrived in the promised land. Push the bottle into the water, and the smoke will be forced up into your lungs. You can do this as slowly or as fast as you’d like, depending on your lung capacity. There is a high chance of coughing, even for the hardiest of smokers, so be warned!