I start with very finely crushed algae wafer - about 1/4-1/2 tablet. They are supposed to be fed a couple times a day, but play around with that since you don't want there to be a lot of uneaten food around.
I also do a pinch of finely crushed freeze-dried brine shrimp - almost to a powder. You can also do very fine small pinched to a powder flakes.
Switch off whatever you want, just not all at the same time. The key is making it small enough for them to eat!
If you want them all or most to survive, you have to keep them in a separate container. I use a crappy plastic one gallon that came with a light and "filter" (air stone) which I use on its own without the under gravel thing that comes with it. It was $10 bucks.
They can co-mingle with the other fish when they are too big to fit in their mouths. If you choose to keep them in your main tank, put a nylon over the intake tube of your filter.