because i didn't have a spare mason jar laying around.
Threads onto snapple bottle perfectly, i made the inlet hole larger than what is seen in the pictures.
fill the bottle with a quarter cup of apple cider vinegar (white vinegar and/or dish soap also work). Wipe some on the top of the lid so it smells.
if you really want to get Alcatrazy, then put tape all around the bottle leaving about a half inch open on the top of the glass. the flies will fly towards the light once they are inside, and they will never find the opening, although the bottle alone is probably 95% effective.
update November 2014:
newer generation of flies in my house seem to not be attracted to the ACV as much. I changed things up a bit and did sugar and yeast, which produces C02 gas and makes them think it is something rotting, this seemed to catch a lot more flies. just have to change fluid more often and be careful they dont lay eggs inside the bottle.
100% fill works well, so the thread ears are strong.