My house is filled with drain flies. Need step-by-step instruction to locate the breeding area?

Process of elimination. First get some household bleach and put it into a spray bottle. Take this and spray about 5 squirts in every sink in the house. Be sure to spray the overflow openings in the bathroom sinks. Do the same in the overflow openings in the tubs. Place a banana peel on an open space and get rid of the flyers with either a vacuum cleaner or use a soap mixture in a spray bottle to kill the ones in flight. Hope this helps! When my husband and I had drain flies in our apartment we tried pouring every chemical we could think of into the drains. Bleach, tub & tile cleaner, CLR....everything. When none of that helped we had to get our landlord and a plumber involved. The plumber explained to us that drain flies lay their eggs in the film surrounding the inside of the pipes and water/liquid will not destroy them until they hatch so often when you get rid of them for a few days or so they come right back. We had to get a crack in the pipe repaired and professionally cleaned before we finally got rid of the problem. Good luck!