Pick someone to practice on who trusts you. Make sure that this person knows that you have no plans to hurt them or actually hit them across the face.
Practice in an area where you have plenty of room away from furniture or anything that can get broken. In a yard of soft grass is preferable.
Face downstage toward the audience with the victim of the slap in front of you at a slight diagonal so they are facing upstage away from the audience.
Make sure you are standing at least three feet apart. Check this by taking one step forward and wave your slap hand forward back and forth in front of you without hitting your partner.
Keep eye contact with your co-actor and start off with both hands at your side. Reach up with your slap hand to the side of your head with your elbow bent at a 90 degree angle.
Swing your hand forward to the side of your partner's face. Since you tested the distance between you there is no chance you will accidentally hit your partner so swing away. You can have a sound effect onstage to complete the slap.
React to the slap across the face if you are the victim. Reaction is important and can make all the difference.