There's no easy way to do this but a good seam guide is invaluable.  I always try to make a point of doing something like this in my products because it's damned difficult.

Seems like you have the right idea; make two copies and line them up.  Scaling sometimes helps if it's a repeating pattern, because you can ensure the two ends fall together properly where they meet.

Something which may help is to paint a white stripe round the arm, showing where the tattoo will fall.  You will then have a much clearer idea of any linear distortion you will have to account for.  As for the seams, though, well, that's where patience comes in. :)

Coppula eam se non posit acceptera jocularum.

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