Hello spark. You have already been given some great advice but I will throw in my 2 cents worth. I learned flogging out in the clubs just watching and mimicking. It's pretty easy. For the single tail I took lessons from a whip master and he wouldn't let me take aim at a person until I could hit specific spots on a dress form (the things seamstresses put clothes on to fit). They are difficult to control and there is a real danger of hitting an eye, ear or other sensitive bit.

Personally I find the butt area more difficult to hit accurately if the "victim" is standing. If that is position you are going for, I would stick with the back.

I wish this image was in English, but I think you will get the idea:
https://www.blackangelsl.net/wp-content/uploads/2014/08/Onde-bater-500x600.jpg