How can I build muscle after anorexia?

Weight restoration. No matter how "healthy" your diet or how much protein you ingest, you need to restore your weight and then worry about weight bearing exercise to restore your muscle mass and bone strength.
Good nutrition! Seems obvious but you can't effectively build muscle without two primary components: nutritional building blocks (good diet) and stress on specific muscle groups (exercise).
Muscle building. First & foremost, restore your weight to normal range for your age & built.