Defeat an Enemy, then swallow an enemy by standing next to it by pressing Q. Go into the options menu "other" and set "Digestion Factor Speed" to max. When the poop icon appears press C to poop out a Phago-Crystal with the type of enemy appearing on it. Press U to put it in you. Then wait or go to sleep. A birth icon should appear press Shift and R at the the same time to give birth. That's one way to do it, or you can just spawn then in on the Tab menu.