My rules I try to go by before doing head swaps:
1.As long as the camera is at the same angle in both the shots, and the lighting hasn't changed drastically, it can be pulled off pretty simply. These are the 2 shots I'm working with.
So I'll pick a base shot. I'm using the one where everyone is smilling but Miss Chilly there.
On the other image, I'm going to select the polygon lasso tool and cut out the good section of the photo I want to insert. Can you see the "marching ants"? The little white lines outlining part of the image. That is our selection. I will then copy and paste that selection on to my base. (I could have completely cut her out by herself with the polygon lasso tool, but to make it even easier, since Miss Peachy to the right of her looks great in both shots, I'm going to cut this out in one giant chunk, which will require much less blending, and make this really easy and fast.)
Which brings me to another rule that I go by.
#2 Cut in as big of chunks as you can. This works for me. Obviously it's a case by case basis, but overall I've had the best luck going this route.
This is to show you just how large a chunk I am pasting onto the base image. Another rule I follow is:
#3. In using the polygon lasso tool to cut out your image, try to follow lines that are in your image or any implied line. This makes blending really easy. I followed the line down the center of the window. Again, if I had to just cut her out alone, and leave Miss Peachy on the end out of it, you can look above her and see exactly which lines I would include while cutting her out. There is a perfect line to the right of her head that goes right on up to the top. I would have chunked it out exactly that way.
After I have copied and pasted the selection onto the base image I carefully match up the pieces. (copy and paste keyboard shortcut, command c and commad v) Now I'm working with 2 layers. In the layers palette, I adjust the opacity to around 50% or so. This allows our selected piece to be transparent. Notice that we can now see through it. This allows me to line the piece up to the image underneath. I want my image to line up in the center. Her blouse, and the hand on her shoulder. If the outside doesn't line up perfectly, I can easily crop that tiny bit off. Once I have lined it up, I turn the opacity back to 100%.
Next, in the tool palette, you want to select the eraser tool. Make sure it's a medium sized feather brush. With this one I started at 65% opacity and I adjusted it up as I went. Usually I end up at 95% or higher depending on what area you're blending. The shirt I blended at a lower opacity. (Also, when blending greenery you can get away with a lower opacity.) Next tip: Before you flatten it, look really close for stupid mistakes. I ocassionally forget to do this. Sometimes I turn off the bottom layer so I can see where I have erased. That's a good way to discover glaring errors that were totally overlooked. Okay we have none, so next step...
If it looks good, I just flatten it, rename it and done. Next I email the file to whichever friend asked for the photoshop favor. Now my brother can finally post this photo on his photo blog of his internet friends who he met in real life. He even, married one of them. (Miss Peachy on the far right.)
If these tips helped..yay! But if you are now dumber, than before you entered this blog, I apologize. They're not all winners.