I'm actually working on a tutorial right now for Clipping Paths (the greatest use for the Pen tool), which is what I'd use for photo manipulation. Manips are very difficult, and I recommend using only promotional stills/production images because they have a higher resolution. Regular screencaps often don't have the quality to pull off a great blend. I'd be willing to take a look at the images you wanted to put together, and if it's possible, I could put together a tutorial for you using those specific images?
I'd like to recommend that you use a clipping path, simply because it's the best tool available- and it's in each version of Photoshop. While CS + have some amazing tools, not everyone has access to those versions. I had CS3 for school, but would use 7.0 at home, and a lot of the techniques I learned weren't compatible with earlier editions of the software. Do not use magic wand. It's also available in every edition, and most photo editors, but it compromises the line quality between the subject and the background.
I would like to stress the importance of picking proper images. Below are two promotional images for Supernatural and Point Pleasant. While it's cool that they're both taken in churches, it's more important that they were both taken indoors, and the image quality is similar. Elisabeth is back-lighted, the boy's are being lighted from their rights, but with careful selection, the harsh white around Elisabeth's arms can be removed, and her shadow can be rotated.
My first step is to always drag both images into the same document, and change the opacity of the top layer to something low... 30-40%. This way, both images are visible, and you can align them to your liking, and then scale them in proportion to one another. Scale is the second most important thing in photo manipulation, and where most people make their mistakes.
Then, I return the top layer to 100% opacity, and begin creating my clipping path, which essentially creates a flawless silhouette and selection source. No jagged lines, beautiful curves, no damage to surrounding pixels.
The image isn't complete at this stage. You can create another clipping path to clean up the edges of her hair, or use the eraser tool (and a steady hand) to pick up the smaller details that you missed. Also, You still need to play with the lighting and shadows, Elisabeth's image is richer in color than Jensen's. This can be toyed with by duplicating the Supernatural layer, and changing the blending (the little menu where you change layers from "Normal" to "Soft Light", etc). Also, you can back off the color of Elisabeth's image with the Hue/Saturation sliders. Keyboard shortcut: Ctrl + U
Play with the sliders, until the images have the same tone, and familiarize yourself with them. I use them all the time for color correction, and couldn't imagine making icons without them.
Anyway, those are just my tips for getting started. I wish my generic Clipping Paths tutorial was finished for you, but if you wanted, I'd gladly take a look at the images you wanted to put together. Like I said, I don't mind putting together a smaller tutorial for your use. <3
Hope this helped! If you need clarification on anything, don't hesitate to ask.