How do you replace a character with the model of your choice?

if your talking about custom bodies, then you basically need to make a fake "torso" and weld all the parts of your body to it. You then use CFrame to make your own animations. It can become very tedious and somewhat complex. If you can find an uncopylocked version of mastrj's Naruto Ultimate Ninja Storm Generations, then you learn from his code. He was one of the first, if not the first, to even come out with these "custom bodies." Although his code is three or four years old, you can still learn from it and make better custom bodies. Hope it helped!