Lynn already gave you good advice. Myself would let media cool down to room temp before adding the yeast (live, not brewers) grains (6-10); then the excelsior; and finally the flies.

I do a variation of Lynn's method and use an empty 32oz container to pour flies into it; ensure got like 50 for D. melanogaster or 100 for D. hydei; and then pour them from there into new culture. Label container and you are done. Good luck !