What I’ve done, in the past is to make a series of ‘registration’ marks on a new layer, with a transparent background, over the skin texture image and temporarily merge the layers (saving it under a temporary name). Then apply that texture to the model. to see where the marks fall. Then split the overlay image (tattoo) and align it to those marks (again, a separate layer)...repeat the merge and test again. When satisfied, I discard the registration layer and merge the overlay, saving it under a different file name something like ‘orginal_skin_tat.jpg’ (did this for a number of Morrowind skin textures…never did release that mod pack…)
Is that the easiest way?
Probably not, but with a bit of fiddling with placement, it works.