All you need is download tattoo photoshop brushes and your photo.
For this tutorial we will use this photo:
Download collection of tattoo brushies. You can download them here. You will get excellent collention of photoshop brushies for tatto:
Extract this brush collection to brushes folder: Presents - > Brushes.
Now open your photo with photoshop:
Go to brush tool: (B). On brush tool menu take on of tattoo brushies that you are download:
Now create new layer: Shift + Cntrl + N, call it Tatto and draw your tatto. Now we got 2 layer:
Now press Cntrl+T (Free transform) and put your tatto to than body place where you whant it will be. And make size of tattoo what you whant, to look like it is original:
It is olmost originaly, but not complete.
Now we go to: Edit ->Transform->Warp and modify your tattoo to be realistic:
Here you need your imaginashin to transorm tattoo what ever you like.
Now it is final step. Go to tatto layer blending mode and change it to multiply and change opacity to 50% or what ever you like, to looks like real tatto:
And here is final result:
Maybe it isnt perfect, but realistic of your tattoo depent of your imaginashin. So be creatif and try yourself. Good luck.