As little pressure as possible, and do it in the sink (etc) so that the surface to be drilled is just under water. Bits that are designed to handle glass generate a LOT of heat, so you have to keep water moving over it to keep the glass from cracking from the bit itself. Glass bits also generate a lot of glass particles in the air, which you really do not want to inhale, and drilling underwater should contain most of them.

If you're really worried, you should go pick up some random glass bottles and practice a few times. Keep in mind that any seams/bends/etc in the glass will probably be weaker spots less able to handle stress.