Basically you need to start with your two images opened in a program such as Photoshop.
You then cut out the head of the person you want to use using a cut out tool.
You then reseize the head so that it isnt too big or too small for the picture you want to put it into.
Then place the head and do some cleaning up with the paint brushes, smoothing tools, etc to make it look right and make sure all the lighting looks ok.
See here for a tutorial:
http://www.absolutelybananas.com/200...asy-steps.html
http://www.tutorials-photoshop.com/p...eads/index.php