Fruit flies can be a pain in the kitchen, but with a good trap it's surprising how quickly you can get rid of them.
The steps are:
1. Put bait in the bottom of the container
2. Cover top with plastic wrap
3. Secure with rubber band
4. Poke holes so flies can get in
Step 1: Materials
1. A container
2. Plastic wrap
3. Rubber band
1. Something pointy
Step 2: Choose a container
I like to use disposable plastic containers because the thought of cleaning out a fruit fly hotel does not appeal to me and I'd rather just toss the whole thing at the end.
Step 3: Bait the trap
Generally this is whatever the fruit flies are hanging around. Some good standbys are wine, old fruit, bananas/banana peels, and orange juice. It's usually desirable to have some liquid in the bottom so they drown instead of setting up families.
Edit: I've been told cider vinegar is also a good bait. Also, adding a little bit of dish detergent* will encourage drowning.
*Be careful with adding detergent, the green apple detergent I have right now scared all the flies away. A second trap with the same bait and no detergent worked great.