After the woman gave me the original starter shoe box full of mealies, I screwed up & lost them all. I found that actually I need 2 plastic shoe boxes because when I clean the box, I found that I need to save the old food substrate because it's always full of unhatched eggs. I threw that away & that's how I lost them all. I went to the pet store & bought a new box of mealworms then started over.
I just throw them in a mixture of cornmeal, oatmeal, whole wheat flour & sometimes powdered milk to give them calcium. Kind of depends on what I've remembered to buy. Then I leave them alone but not in the refrigerator, I leave them out at room temperature.
Once a week I make sure they have some sort of fresh moist vegetable, celery, cucumbers, potatoes, carrots all work. I stay away from fruits because those fruit flies appear like magic. lol
I'm not sure exactly how long it takes but it's just a matter of weeks, they'll morph into a pupa, then emerge from that as a beetle that doesn't fly. The beetles will lay the eggs & in a few weeks the substrate is writhing with fresh meal worms.
Eventually they eat up all the food substrate, that's why I change it. I bought a giant sized tea strainer at the Dollar Tree & I'll use it to sift through the old substrate about once a week to get anything newly hatched out of there. In a month or so that box will end up with nothing in it but old food, then I throw it away.
I have a never ending supply of mealworms, I like that because I have 7 turtles that'll eat them & it does run into some money if you have to keep buying them all the time.
It's kind of gross at first but you get used to it & when you see how much money you save, it's easier to get used to. 
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PHRatz