there are a few ways you can do this so bare this in mind.
the last time I ordered flys I just put them all in a deli container to hatch in my warm (76F-78F) bedroom.
they need to be gutloaded...
mix together 1 soft boild egg, 1 tablespoon honey, 2 tablespoons of plain yogurt (think calcium), and enough cruched up cereal (multi grain chereos or kashi heart to heart-the kind that looks like chereos) to make it a little thick.
when the flys hatch just put them in the fridge for a min or two, then take them out and smear a little bit of the gutload on the top side...
it will drip down a bit and the flys will re-animate and eat the stuff like crazy.
feed every day as it drys up inside the countainer.
put them back in the fridge before you want to feed them off, and then just dump a few in the bottom of the cage.
Harry