Quick notes this tutorial works best with high quality images, also try to find a tattoo's in which the skin matches your skin you will be working on, that will give the best results, here is a .psd of the sources for you guys to practice on. Lets begin!
Lets first look at this comparison I made below, the first image is the basic cut and paste technique with an overlay blend, no way you're going to fool anyone with that. Now lets take a look at the second image, the second image uses cutting, feathering, blending, and sharpening techniques to give it the look that the tattoo is really embedded in the skin, so much so that you can see it literally pop out! Alright, lets begin.
1. Choosing our source images.
Here we need to find two images which we want to work with. It can be yourself, a friend, or someone you want to see how a tattoo would look on them.Then find a picture of the tattoo you want, it's best to take a tattoo off a picture of someone who actually has a real tattoo, thus giving a realistic look because the tattoo has already been embedded in the skin for you. As an example I've chosen my two images as the the following.
2. Placing the tattoo source onto the main source picture.
Take the tattoo source and place it onto the main source (crocop) this will create a new layer. Now on the tattoo layer use the Free Transform option by pressing (CRT+T) now position the tattoo where you want it to be, and rescale and rotate it to fit the current angle.
3 Deleting unwanted areas to fit shape.
Now we need to get rid of that extra piece of junk from the tattoo image so it will perfectly take the shape of Crocop's shoulders, first off we will want to make the tattoo layer invisible by clicking on the eye right next to the tattoo layer. Then select the polygonal lasso tool and follow the shoulder area like the image shown below. (I know you can use the magic wand tool to get rid of it but for me it's a better practice to use the lasso tool.
After you have selected the perfect shape around his shoulders make sure your selection is still visible then click the tattoo layer and make it visible again, then press delete. Now you can see the tattoo is starting to take effect by taking the shape of his shoulder, were almost there!
4. Feathering tattoo layer.
This part is very easy and simple. select the magic wand tool (w) and while on the tattoo layer click an area outside the tattoo so that the whole area outside the tattoo is selected.Now go to select > modify > feather and enter a range from 8-10 (for smaller images a range from 3-6 is good) after the feather is set press delete twice, now you will notice the edges of the tattoo is now very soft on the edges making the tattoo ever more realistic on the body.
5. Darken overlay on tattoo.
Select the tattoo layer and on the layers option, drag down to Darken, now this makes the whole ink park of the tattoo appear while the rest of the color disappear to give the effect that's it's actually on his skin.
6. Touch up.
Well were basically finished, from this point you can just play around with it anyway you want. I adjusted the curve, lowered the saturation, sharpen the image, and matched the color. Here's the final product. Hope you enjoy!