I did this by plotting out my forearm, taping tissue to it, and drawing on roughly where I wanted the elements to go. Then I scanned that rough sketch and rendered it in Illustrator. It changed a fair bit as I worked on it, from where the initial sketch went. Time consuming yes, but I think trying to do this by hand would have been near on impossible.
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Left Forearm Tattoo
Tattoo design for my left forearm. Working on one for the right forearm, which will be in a somewhat similar vein, but different.