How do I know if I'm doing a pose correctly?

I’m doing yoga at home, following along with YouTube videos. But I’m really puzzled. How do I know if my poses are correct? If I lift my head or turn to look in the mirror, my posture gets all crooked. And without the mirror, I have no idea if I’m doing the moves right. Especially with poses like Downward-Facing Dog, where I can’t really lift your head to check. 😓 #Yoga