You need to put the nymphs in a container that you can seal (a cup, plastic box, whatever you want to use). Cut some ventilation into the container, usually 2 full sides of a container will be sufficient and then glue some paper towel or VERY FINE mesh to finish up. If you use mesh make sure that melanogasters can't pass through it, also take note that melanogasters become "smaller" once their digestive system is empty from not eating. If you use paper towel pay close attention to if there is enough ventilation. If the container's air smells stale or really humid when you open it, ventilation isn't sufficient. The most obvious sign you'll see is mold starting to grow, in which case you will need to add more ventilation. To feed, I take the fly cup (I hope your vendor sold them to you in a similar packaging method), smack the lid and the sides of the cup a couple times. Don't slam it too hard, the pupa might get loose, but smack it hard enough for the adult flies to fall to the bottom of the culture. Then when you open it, you won't get a couple hundred all over your floor. Then just open and tap however many flies you want into the nymph container. You can also get creative and transfer the flies into a testtube first using something like a funnel and create a "feeder hole" in the nymph container, that way you don't have nymphs running around crazily whenever you feed. It'll be real messy the first couple times, but you'll soon get the hang of it : ) Good luck!
Ghost ooths should hatch inbetween 4-6 weeks. There will be some extreme ooths that lie outside of that general timeframe though.