"The end goal of a good retoucher is to make their work appear invisible while creating some alternative reality. In this case we're going to look at one method I use for adding a tattoo to an image while making it look like it's really a part of the original photo. This method involves placing an Illustrator file (the tattoo source image) as a Smart Object, using the Free Transform/Warp Tool, adding a Gaussian Blur (to adjust the 'Focus') and then finishing it all off with a lighting effect created from a Channel Pull. Ready to begin?" Dennis Dunbar

Here is a link to the written version of the tutorial Adding a Realistic Tattoo the Easy Way with Photoshop and the video version is below.

Adding a Realistic Tattoo the Easy Way with Photoshop