Is it important for your writing? Or are you trying to shoot this scene?
There are a number of ways it can be accomplished. The most often way is to frame it so that the victim's face is in the forefront, the arm or body of the assailant is in the background. We can't actually SEE the whip landing. The action is performed, falling well short of the victim's back, usually slapping on some object interspersed. The actor 'reacts' to the sound. Cut to the reverse angle, the makeup and FX are applied, showing the stripe. We see the assailant wind up again - cut to the front.
These days, it's possible to CGI the entire action from the back. Simply put a whip handle in the assailants hand, have them 'whip' at the back - someone cues the actor's reactions - the whip, stripe, blood and sound are all added in post.
One should NEVER attempt to 'whip someone' lightly - with a real bull whip.
Notice in the first few flogs in the classic scene from GLORY - it's not a 'bull whip' - it's a flogger. Likely made of felt. The few strokes are used to set the action. The camera doesn't dwell on it. Instead - it's all in the sound, and the face.
http://youtu.be/nDOPJawut4Y
In this old scene from Rawhide - they use a bull whip. Again, one stroke to establish the hit. If you look carefully - the whip goes over the indian's right shoulder. This is something a TRAINED whip expert could do - but not something an amateur should try. The end of the whip is moving faster than the body. The end passed over his shoulder - but COULD have hit him in the face. Very dangerous - but typical for fifties era stuntmen to do.
http://youtu.be/9LxwPSgPnOo
Again - the drama is in the sound, and the reactions on the faces.
If you're WRITING the scene - just write the drama of the moment. The actual camera work and stunts will be worked out by the director and stunt co-ordinator.