Dont be afraid of doing the tattoos! Practice does make perfect and start out on some paper till you get the hang on what patterns work for you. I found it surprisingly easy once I got started (not that my results were fantastic but it was by no means as hard as I imagined).
Before you start focus on what color you want. I recommend a good contrast color so the tattoo does not blend into the skin tone. If your skin tone is rather "warm" eg using a dwarf flesh like color mixed with other, then the tattoo could be blue'ish to create a contrast in the colors. Of course too much contrast is no good either but hopefully you get the picture : )
Also I did my ogre tattoos but first making up the shape in a darker color (eg dark blue) and then highlighted with a lighter blue/grey (using grey here makes them more neutral and I would do this rather than using another blue). BUT do the highlight inside the darker color so the lighter color is 'inside' the tattoo.
I hope this all helps you getting started and I found it is: Easy, hides sloppy lack of details on the skin and adds a unique part to the otherwise rather similar ogre bodies!
Edit:
Have a look in my gnoblog in my latest post:
http://www.ogrestronghold.com/forum/index.php?topic=26851.0The guys on the upper right hand side has reddish tats but these blen too mcuh into the skin sadly - they took longer time to make and the results is no good to me, but at close range they do look good : ) Just not what I wanted. The more tribal ones was faster and I think they work better - mind you, these models were my practice models for tattoos, so it is really just getting started : ) ASk away if you have any questions to what I did.
Edit2: And dont do the tattoos too small or use dots, they get grindy as you also can see in my link : )