Storyboard the fight. For those who are new to making movies, storyboards consist of rough sketches that allow the cinematographer and director to shoot a sequence of shots that make up a scene. The punch cannot be sketched out in isolation. Develop the shots that lead up to and flow from the punch. For instance, determine whether the punch knocks someone to the floor. Additionally, create a plausible context for a fight. A nasty insult or brandished pistol would be just cause.
Rehearse your actors. For example, the puncher might be trained to throw a swift left hook but with minimal impact upon contact. In boxing, this technique is referred to as "pulling a punch." The actor to be punched should react as if she received a genuine blow. If the punch is supposed to knock her to the floor, make sure the actor falls in such a way she does not risk injury. Facial expressions are also important. The puncher should display emotions that make the act of violence plausible. The victim should display a realistic degree of pain, or, alternatively, anger in preparation to retaliate.
Scout a location for the punch. A china shop with priceless goods would not be a wise choice unless your script calls for such a place. Wide, open spaces such as parking garages or cornfields give actors plenty of room to stage a fight. If possible, secure permission from whoever owns the location. Even low-budget films require shooting permits.
Shoot the punch. Film several takes from different angles and perspectives. If the fighting leaves your actors disheveled, clean them up before the next take. Also, let them rest a bit if needed. Intense physical action can be draining for many actors over the course of multiple takes.
Edit the footage of the punching sequence. Do not leave any space of time between the throw of the punch and the victim's reaction. Using multiple takes obtained during the shoot and find the best pairing of the punch and reaction shots. Add sounds of the blows and their impact in post-production to maximize the realism of the sequence.