Video Length: 10:19. I do give a lot of detail here for those who are quite unfamiliar with Photoshop, but if you are already familiar with the software this should be a breeze.
Photoshop Tutorial - How to make a clipping mask.
This works on versions 4, 5, and 6 of the Adobe Creative Suite.
In this video I hide* my selections a lot because it is easier for me to work with hidden selections, but it is optional. You will get the same results even if you don't hide your selections.
*To hide/show selections:
On the mac: hold down OPTION + H
On windows: hold down ALT + H
I hope you find this useful.
Feel free to comment or to ask for a specific Photoshop tutorial.