Summarize this article in one sentence.
No meat or dairy.  Check on it every few hours. There should be a swarm of fruit flies in 3 hours or so. First, snap a lid on the jar, and then turn it upright. You can then follow the directions below to culture them, or feed them right away to your pet.

Summary:
Put scraps of old fruit/vegetables in a large jar. Set the jar near a trash can or near a compost pile (without the lid, but make sure you've got one that fits tightly). Leave the jar there for several hours. When you've got a lot of fruit flies in the jar, you'll have to work quickly.