>>384811143
You shouldn't mess with the UV unwrapping, as they'd be unwrapped already.
Here would be my steps.
What you're seeing in the video is texture atlasing, basically taking all the different textures and putting it all on a single image, which is slightly different than what you're doing.
Shift+D to duplicate the body mesh. Then you'd create and assign a new material for the duplicated mesh. Then just bake from old to new, and once the image is made, save the image then use that image as the base texture for the new material.