This is a requested guide. I don't currently have a fruit fly problem but I learned this when I lived in an area that did have a lot of fruit flys.
Take a jar (cup or bottle) and put some fruit in it. Bananas works great (I didn't have any so this ball represents fruit). Tightly cover the top with plastic wrap and rubber band it in place.
Put a small hole in the plastic wrap just big enough for fruit flys to get in. Leave this jar (or as many jars as you want) near your fruit.
They'll be attracted to the unpeeled fruit. You'll find it full of fruit flys. I like to use a jar so I can just screw the lid on it. They don't live long anyway. Just clean it out and reuse it.
These next few images will just be different variations of materials to make the trap out of. They all work the same way. Some use fruit juice or honey instead of fruit.
I hope this is helpful.