You need to use the skin builder tool to do that. It requires a lot of Bit map image converting and resizing. Very hard to make. The skin making program wont even run on most systems as it is for windows xp. It's really hard to do.
btw i feel this a good time to remind you guys that sea of thieves was made by rare, who also made DKC1-3 and DK64, alongside banjo kazooie and conker's bad fur day