1) First open the images you're going to use. In this case I'm using this Buffy promo and this picture of Hilarie Burton.
2) My base was a little grainy and I didn't like that. So I went Filter > Noise > Despeckle. I did that twice to get the desired smoothness I wanted. Depending on your base you may need to do it more or just skip this step.
3) I wanted to make the base brighter so I went to Enhance > Adjust Brightness/Contrast and adjusted the brightness to about +20.
4) Now I went back and resized Hilarie's picture so her face was about the same size as Buffy's. It may take a few tries before it's just right.
5) I went and selected the Selection Brush Tool, for PS Elements its located under the lasso tool. If you don't have this tool just use the lasso tool or any other selection tool you're comfortable with. With my selection tool I selected the area I wanted to use on the manip. In this case, I just selected her face. Sometimes I have to select the hair as well depending on the picture I'm using.
6) Then I brightened her face some. Adjusting the brightness to +20.
7) Once I was happy with the brightness, I dragged her face over to my base picture and arranged it so it's the way I want it. You may need to rotated it some depending on your picture.
8) Duplicate your base and drag it to the top. Make sure that the duplicated layer is the only one visable. It should look like this.
9) Take the eraser tool and erase Buffy's face. Once you're happy with that. Reselect, make visable again, the background image and layer 1.
10) Hilarie's skin is pretty close to Buffy's but not quite. Instead of changing the brightness & contrast on her face again I selected Buffy's face, neck, chest, and shoulders. Adjusting the brightness to +20 and the contrast to -20.
11) Merge the layers, sharpen, and you're done.
Icons from this manip