I'm not really much of node-wizard myself ... more like the exact opposite haha .. but well, I guess that you would connect the Blender nodes Input_1 with the next Blender node, and so on. Input_2 of each Blender node then will receive the Image map, you want to blend in. The Blending value is 1, with an attached mask image of the tattoo/decal.

Pretty basic, how I'd star this experiment.

Perhaps instead of a mask, a math function will/could work, too.

... speaking of them (math functions), I wonder what funky stuff all the different math functions can do, when plugged into the Blending value of the Blender node.

There's still so much more to find out, still ... lol