Help designing skins

Help designing skins

When you draw a skin a program like Gimp, how do you ensure that the parts of the image, such as the 'head' part, go to the right parts of the unit?

If you are editing a skin foir an existing model, you don't need to change anything around. Everything is already in place. However if you are skinning a costom model or one built from scratch, you'd have to assign how the skin is wrapped in the model's 3D program. (i.e. which part of the skin layer is attached between which verticles).
-VGsatomi

Ok, So I made my own model and I want to give it a skin I made. How do I tell it to put each part of the skin in the right place?