Hi, Bruna! I'm a Brazilian physical therapist (or physiotherapist) too! I think you can say "lie prone" / "lie on stomach" / "lie down on your stomach / (for deitar de bruços) and "lie belly up" or "lie on your back" / "lie down on your back" (for deitar de barriga para cima). You should try this sentences with your patients and tell us about your results! Good luck and keep in touch!