Step 1: Inverted Cone Fly Trap
To make it, cut off the top of a soda bottle and insert it into the remaining part inverted. Tape or hotglue it in place. Put some liquid bait in the bottom.
The flies will fly in and will follow the walls back up, not finding the way out.
Instead of the plastic cone some people use a paper cone,
a plastic bag with the corner cut off,
or a piece of saran wrap with a small hole in it.
some people use these Alternative Baits:
juice or sugar water'
sugar water with yeast
apple cider vinegar
for other types of flies: meat, fish, canned cat food, or whatever the flies seem to like.