Excellent link to SilkWorms 101 thread.
Follow the info on that thread and do not leave them in the petri dish they come in for long. If you only order a couple in a to-go cup, leaving them there as you feed them off will be fine. But if you order 100 or more in a petri dish, they will quickly over crowd each other. Remember to keep proper temperatures and dry out chow (if you are not using leaves) in between feedings to prevent mold. Mold will quickly kill silkworms as will contamination if you do not wash your hands before handling silkworms or food.