Basically you boil up the right amount of water, toss it into a bag or pouch containing your dehydrated food and set it aside for a few minutes. Especially by insulating the bag, you can prepare dishes not specifically designed for this technique. I understand some forms of pasta (small diameter types) will cook up very nicely with this technique.

Google "freezer bag cooking" for lots of info. Lightweight backpackers are fond of 'boil in a bag" because the cooking apparatus is simple, light, and cheap/