Making tattoos in SL is relatively easy, but if you haven't tried it before then a manual describing the process from a to z is a good beginning.
The hardest part is actually to find good tattoos, make one or processing RL photos to tattoos (I'm working on the manual :-).
The GNU product: GNU Image Manupulation Program (GIMP) is highly recommended for for amatours and semi-professionals. It's easy to use and has an excellent Help Manual
So here is the manual "Make SL Tattoos Manual": http://www.textuploader.com/5bzyi
Eli (iamEli Resident)