Step-by-step: Create a realistic, complex tattoo
Break elaborate designs down into parts, and use repeating elements (like the geometric shapes here) to make it look cohesive. Get an idea of where each part goes beforehand by sketching out the tattoo on the portrait first. The tattoo doesn’t have to be perfectly clean, since real tattoos are done by hand.
Place your flat design where you want, then Transform>Distort its layers to match the perspective. For simpler curved surfaces (say, across a calf), Transform> Warp might do. For greater control, I use Liquify. Click Advanced Mode and Show Backdrop with the settings shown, so you can see the background while liquifying.
For this subtler, aged tattoo look, I have a layer in Overlay mode and others in Soft Light and Multiply. I alter the Opacity to blend with the skin and use Gaussian Blur, because an old tattoo will lose its crispness. For a new tattoo, select Blending Options and add Soft Glow set to Multiply for a red outline of irritated skin.