I can't point you to a specific tutorial, but there are unimaginably many Photoshop tutorials on the net. Should be easy to find one. I don't remember where I found it, but I once downloaded a spectrum pattern, you know, like paint samples, all little boxes, of all the various possible skin shades, from white to Latino, to Asian, to Indian, to black. It's very helpful to have that reference. The rest is adding slight noise for pores and making the skin color uneven, because skin is not one consistent color. I add scars and tattoos by applying the image in layer mode and selecting multiply or overlay and adjusting the opacity. Makes it look more natural to me that way. The scars, I blur and blend the edges to make it blend in better. Wounds are harder, those take much work. Freckles are the hardest. To make the skin blotchy, find a grunge map that looks nice and random and blotchy, black and white. Apply that in multiply or overlay mode. Take the opacity down to zero, then start boosting it up until you achieve a nice blotchiness that isn't obvious or overbearing.

Talent is God's gift to you. Using it is your gift to God.