They're very small, nothing like a normal sized fly. You just open the lid, quickly shut it, and shake it into the water. Of course, some do tend to get loose. But they normally stay on the water, I keep the hood on just in case. You can see the movement on the water, and they'll detect it. My Julep was scared of them at first, but Kaida was all into it from the start and ate them like her life depended on it! ;)
PetCo sells them, too. It's by their bettas. "Betta Bugs" are what they're called, I believe.
Kaida- CT Female 2.5 Gal.
Julep- HMPK Male 2.5 Gal.
Fandango-HM Male 10 Gal.