When you sob, you always inhale deeply, so that it looks like you are sobbing. You should use typical sobbing body movements.
Extended information:
Crying skills
Think about the saddest thing, the humblest thing and the most painful thing. Divert your attention and situation, and let yourself suffer quickly.
Actors should go deep into the feelings of the role at that time, which is usually called entering the play. Let you really feel the emotions in the role, you will burst into tears, and then take real actions to let the audience touch the soul.