I have two photos I want to use to create a double exposure image. How can I do this with Photoshop CS5?
If the double-exposure effect you're going for is the traditional multiple-exposure look of images overlaid on each other, you can simply place each image on a separate layer and adjust the transparency of the top layer(s) or multiply the layers.
If the double-exposure effect you're going for is the one-image-inside-the-head of a portrait, you place the portrait on top of the background. Then select and erase the bits that aren't the head on that layer (most of the video tutorial shows the various selection tools you can use). Then, with the same selection in place, do the same erasure on the background layer. Then multiply the layers (alternatively, you could adjust the transparency of the top layer).
Alex Wise had a detailed video tutorial with PS CS5 on his blog here: http://www.alexwisephotography.net/blog/2014/05/05/how-to-create-a-double-exposure-in-photoshop/