Using a PSD,

1. For starters, download the PSD that you want. You can find some PSD's here, or at Foranga.
2. Open it up on photoshop. You would open it like a normal photo. File>Open, and locate where you saved it at, and open it up.
3. Then open your other photo that you would like colored.
4. On the PSD picture, highlight ALL the layers on it. Every layer, expect for the base. Like this.
5. With your 'Move Tool' selected. Drag all the highlighted layers to your nonpsd picture. Here's an example.
6. Now your photo should be colored. You have mastered using a PSD :)

If you're wondering how to make one, then just open a regular photo and color it. But do not flatten the image! Once you have your color, go up to File>Save As, and save it as a .PSD file. Here is another example.