The person on the other side must be able to see you clearly. Be sure you're in a room with a good source of light, like in front of a window, for example. Just be careful with weird lighting, such as from above, below, or only on one side of your face. It can make you look strange and make it difficult for the other person to see your movements and expressions.
Watch the video below from 3:55 to 5:30 for more tips: