Summarize this article in one sentence.
While any glass bottle can be turned into a water bong, some bottles are better than others. Bottles with a long neck and a wide base work best. Getting a clean, perfectly sized hole for the stem is easiest with a bottle that has little curve on the upper slope where the base connects into the stem. You'll need a 3/4" diamond drill bit to drill through the glass. Despite having the word "diamond" in the name, these usually only run for about eight to twelve dollars. You might have to order the bit online so make sure you don't leave this step for last. You'll want a 14mm glass bowl and downstem. Choosing the proper length for your downstem is important. Measure the distance from the upper curve of the body of your bottle to the opposite side of the base. Acquire a downstem approximately the same length, and definitely no longer.  Glass downstems and bowls can be found at local head shops or ordered online. You'll need a rubber grommet with the following measurements: an outside diameter of 31/32", an inner diameter of 1/2", and a 3/4" groove. These can be found at supply stores like Home Depot, or can be found online.

Summary:
Pick your bottle. Get the proper drill bit. Acquire a downstem, bowl, and grommet.