First we start with a soda bottle.The one in the photo is a 1 L bottle but a 2 L bottle will also work.
Cut the top off has shown.
Invert the top and shown.
At this point you can glue or tape the inverted top in place. You can also add a couple holes at the top edge for a wire or string hanger,as in the following photo.
The bait I use depends on the type of insect that I wish to trap.
houseflies: one egg in 1 cup water blended. (If you use a 2 L bottle you may wish to use 2 cups of water instead of one.) This bait will work immediately and does not need to sit and ferment for it to work.
wasp or hornets: use 1 to 2 cups of Coke soda (not diet)
Place trap away from your house, as after a few days it is going to smell real bad. This will also draw the insects away from your home. Egg wash bait will also attract animals, so be sure to place trap out of reach of dogs, cats, skunks and other animals.
When all of your insects have drowned in the trap and it is full, simply throw away entire trap.