Getting the most out of a seltzer bottle (i.e. getting the most fizz) means being attentive to both time and temperature. Seltzer bottles work by releasing pressurized CO2 into a volume of water where, over a period of a few hours, it dissolves. You simple fill the bottle with water (filtered is best) and put in the siphon.
Drop a CO2 cartridge into a little sleeve-like cup that comes with the bottle…
…and screw it down onto the valve on the side of the bottle head.
This action punctures the top of the CO2 cartridge releasing the gas into the bottle. Once you hear the hiss and a little gurgling you know the job is done, you simply unscrew the cup, dispose of the empty CO2 cartridge (you can recycle them at a local gourmet shop if you like) and screw on the much more space-efficient valve cover.
Give the bottle a good shake to encourage mixing (remember Joseph Priestley?), stick it in the fridge for a few hours and you’re good to go. Probably the biggest factor that influences the solubility of carbon dioxide in water is temperature. The lower the temperature, the more CO2 the water will absorb. Thus you want your water as cold as possible when you gas it, and you want to keep it that way until you’re ready to serve it.
Sadly, because no home seltzer bottle is completely air tight, your home made soda will start to go flat after about 12 hours. All the more reason to make that second round of ice cream floats and be quick about it.