This is a brief explanation of how I manipulate layers in Photoshop. Similar principles would apply if you're using another image-editing application that uses layers. If you're already an expert with layers you can probably skip this page.

If you rendered two images like I do, you'll have two separate documents. The idea is to get them both into the same document. Pick one (it really doesn't matter which) and use the Duplicate Layer command. You'll get a box like the one on the left.

Use the pop-up menu to choose the destination document. If start with the "bald" document, then you'd want to pick the "hair prop" document as the destination. If you started with the "hair prop" document, you'd want to do it the other way around.

Okay, now both layers are in one document, but one of them is probably named "Background" and the other is probably "Background copy" or some such. I prefer to give each layer a meaningful name to help keep things sorted out in my own head.

If you go to the floating window with the "Layers" tab and double-click on one of the layers, you'll get a dialog like the one on the left. You can type in any name you like. Also, if you do this on the default "Background" layer it will stop being a background layer and automatically become a "normal" layer.

You should end up with something that looks like this. Note that I've added a few extra layers on top here, but the main point is that I have my "bald" (No Hair) image on the bottom, my "hair prop" (Base Image w/Hair) second, and all other layers above that.

The "Hair" layer is where I do the actual hair drawing.

In this case, for my Thirteenth Plane image, I have additional layers. The "Fixes & Webs" layer was for drawing in the spider webs (very similar in principle to hair), and for doing some more radical fixes. The "Auras" layer was for airbrushing auras around the candle flames.