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Hi,
This can be quite difficult to do, depending on the images you have, the flesh tones available and the skill yout currently possess. I would certainly say it's an intermediate to advanced thing to be trying to accomplish!
You need a high-res image to begin with and the images of the heads need to be at least as big as the heads in the image you're replacing, otherwise you'll end up with pixellation. It also helps enourmously if all the images have the same background colour and if that background colour is one colour only (as opposed to a photographic background). It's best if the background contrasts with the object you're cutting out...
The jagged edges you're describing are probably due to how you're cutting out the image. Heads are notoriously difficult to cut out. Most of the face is quite easy, but hair is something else entirely! It can literally take hours to cut out a head with detailed, hi-res hair.
Pointers I would give you... Try to zoom in quite a bit and cut on a small scale. Don't use any of the selection tools if they don't give you good results - and most of the time they won't give you a good result on anything organic-looking. Instead try to "erase" or colour around what you're cutting out, using a very small brush. You only want a few pixels diameter unless you're dealing with a very hi-res image. Do it in stages and remember to make short strokes when cutting out, as the undo feature will only undo the last entire stroke you made (the last time you pressed and depressed the mouse). So holding down the button and cutting out most of the image perfectly can be completly ruined by one small mistake.
The rest, I think, comes with experience. Remember you can use blurred brushes and square brushes. the square brushes are more precise and faster acting, but they leave jagged edges. The blurred brushes take longer to work but leave a smooth edge and look more natural.
When it comes to actually joining the heads to the bodies you're going to obviously rescale the heads to fit the necks... but you'll also need to touch up the colours so that the skin tones match and join up the skin effectively. The clone and healing tools will help joining the two together and can blend some of the colours. Try to blend it so that as little of the original neck exists as possible as then the only other skin showing should be hands, etc and won't be as noticeable.
I think that's about all I can give you off of the top of my head. If you have any specific problems then let us know and possibly post the images you're using so we can give pointers. If the worst comes to the worst, I'll try to do something similiar myself and tell you how I solve the problems I encounter as it's always difficult remembering exactly what you'd do!
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