The answer is simple science with a bit of logic tossed in. Things close to the camera image larger than things further back from the camera. When we work the camera in too close, the nose reproduces too large and the ears image too small. This perspective distortion makes people look weird.

Your mental picture of yourself is that view you see in your makeup and dressing mirror. The trick is to reproduce this view (perspective).

The cure is to simplify place the camera further back. This is not easy if you are doing a selfie. Professional portrait photographers use a moderate telephoto. This simple act forces an elongated camera to subject distance. It is this simple act that solves the distortion issue.

As to a round face, the cure is in the lighting. Most portraits look best when the light seems to come from above. In other words, we light in such a way as to simulate afternoon sun. A lamp in this position casts shadows. These are needed because they deliver an illusion of depth. We need this illusion of depth because our images are 2 dimensional. Now that we are paying attention to the shadows, we position the subject in order to achieve a flattering light.

As an example, if the subject has a long nose, we adjust the lamp or the subject to attain a short nose shadow. If the subject’s face is round, we position the subject so that one side of the face is in shadow. This light and shadow enhancement is called “Chiaroscuro”.

So to answer your question: Use a moderate telephoto or simply step back. Also, try to position the subject so one side of the face is shadowed.