I captured several videos from my desktop this afternoon, going through the process of clipping paths and photo manipulation from start to finish. I felt awful that I couldn't speed up the production of the written tutorial, but I figured that a video would be just as helpful, if not more. I'm converting them from AVI now, and I should have a video for you within the hour (Computer willing).
While you wait:
The Clipping tool is just the "Pen Tool" (Keyboard shortcut "P"). Go to "Window" > "Path". It will bring up the path window. In the lower right corner, click the "Create New Path" button, and it should create a new layer in that window that says "Path 1". Then zoom into the image and trace the outline of your subject. Once you have a full silhouette and have closed the open path, right-click on your path layer, and select "Make Selection", accept the settings, and you have your subject in a clean/clear image to move as you like. Below are some images from my unfinished tutorial, but should help you get the gist of things while the video/complete tutorial is in progress.
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"The photos are another problem of mine. How do you line them up, when they aren't the full photos?"
When I don't have a full photo, I do a similar thing: Instead of lining them up at the feet, I resize the top layer using the subject's head for reference.
Then I just move Elisabeth down to where she looks "right". It's all very subjective, but if you play around, you'll get the general idea of what works. Our eyes are trained by magazines, tv, movies... you can always tell when something just looks out of place.
"Also, does it matter if they have similar background, or can they be different as long as they are both indoors/outdoors?"
The indoors/outdoors thing is my personal preference. Sunlight and artificial lighting are so different, and often don't mesh well. The backgrounds can be as different as you like, as long as you can clearly see where the subject ends, and where the background begins.
"What about size? If they aren't the same size?"
If they aren't the same size, I just put the larger image on top of the smaller one, and just resize using either the head or the feet for scale reference. Your Victoria Secret models for example, I dragged the blonde into the brunette's image.