oph3lia - Bink13y at MTS2 gave me this tip on making skin tones 2 years ago:
Start a Body Shop project with a full body mesh of the target mesh (let's assume a Maxis normal male - this would be the mesh used for briefs and bikini swimming wear)
Paint the whole of the
body~stdMatBaseTextureName_alpha.bmp white
now replace the
body~stdMatBaseTextureName.bmp with the skin tone's
ambodynormal~top~stdMatBaseTextureName.bmp (make sure to rename it!)
Now as you make changes to the skin tone - click the refresh view circular arrow.
A UVmap of the meshes will be helpful in getting the approximate locations of where to place things so they meet properly at the seams - but will require some delicate work getting them pixel perfect.
NOTE: CTRL + Z and CTRL + ALT + Z are undo in most software! you can use this to step back before flattening the image and make more changes!
as for laying on the tattoos, it is best if you are using image editing software that is capable of handling layers (e.g., Adobe Photoshop, Gimp, etc) and will scale things by means of a bilinear re-pixelation (may not be available in all software).
I use Adobe Photoshop 7, in it I make one layer of the image and carefully erase all the portions that are not "tattoo", set the blend mode to "colour", make a duplicate of that layer, change the blend mode to "multiply" and reduce the opacity until it looks good.
Dafont.com has some good tattoo fonts that you can use for making your own tattoos
Working from images of real-life tattoos is very difficult to get perfect results since these images are in fact photos of a cylinder with the texture wrapped around it. So I tend to recommend working from the tattooist's art pages rather than the real-life photos of already done tattoos.
Any way - when all is to your liking, you can copy the layers to the various skin tone textures and flatten them. And proceed with your female side using the same technique.