A: I am so nervous about my presentation tomorrow. I hope I won’t fall flat on my face.

B: Relax dear. You’ve spent so much time preparing it. I am sure you will do well.

Note: To fall on one’s face is to literally slip or trip up and land on one’s face, i.e. face first. Figuratively speaking, if someone is described as falling on his face or flat on his face doing something, then they fail — completely and miserably. They fail to deliver. They fail to get the job done. They fail to accomplish the feat, whatever that feat is, and kind of embarrass themselves in the process.