There are a few things you need to know before trying to Photoshop a tattoo on someone. First, make sure that the tattoo is properly sized and that the colors are correct. Second, make sure that the background is white and the foreground is black. Finally, use a black brush to erase any lines or blemishes on the tattoo.
How to Add REALISTIC Tattoo in Photoshop – Fake Tattoo Photoshop Tutorial
Check out How To Rotate Letters In Photoshop?