Insert on a new layer the already scanned picture of a tattoo. Open it firstly in a new window (ctrl+o), mark it out with the Rectangular Marquee Tool and make a copy of it Edit ? Copy (ctrl+c). Get back to the girl's window and insert it on a new layer (ctrl+v).



Decolorize this layer: Image ? Adjustments ? Desaturate (shift+ctrl+u) and increase the layer's contrast, applying next option: Image ? Adjustments ? Curves (ctrl+m).



Insert the picture on the necessary place (in this case we have to put it on the forearm), using the Free Transform selection (ctrl+t) and change the layer's Blending Mode on Multiply.



Just to make the tattoo look more real, we have to create the illusion that the picture repeats the arm's volume. It's possible to do this manipulation by applying the Turbulence Tool in the next window: Filter ? Liquefy (shift+ctrl+x).



Erase now that part of the picture situated outside the skin's borders (Eraser Tool).