How do you create a skin, or; edit one with permission from the owner?

For example, lets say I wanted to create my own skin, with a specific police department, or company? <--- How would i go about doing this?

or; as another example, lets say I have a skin (with permission to edit as long credits are given) I wanted to add a logo, or words. <--- How would i go about doing this?

Find a template or a skin for the same vehicle, open it up in a graphics program (Paint, GIMP, photopshop), draw on it, write on it, stick other images on top of it, then save it and import it back into the vehicle.wtd

This is a bit more advanced

Thanks!