Pauline,
I know exactly what you mean. It’s a common task to paste the missing person into a group photo, or as I have done a few times, replaced the closed eyes or frown of a person with that from another, better picture of that person.
PS has many ways to do just about everything. If you ask 100 photo editors in this forum, you’ll get 99 1/2 different ways to tackle the problem. This is because they have different versions of PS, different skill levels, and different photos to start with.
I don’t know what version of PS you have, but in CS you could use the Color Match command. This works great on skin tones. You could work the issue of fluffy or frizzy hair by using a mask. Remember that it is often best to feather the selection to get it to fit into the new photo. Also keep in mind the direction of the light. All of these issues tattle on you when merging pictures or "faking" if you don’t address them properly.
If you do a Google search on combining photos you should get some good help from free tutorials. One that I started with is at:
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http://www.arraich.com/ps6_tips_combine1.htm>
Scout around and you’ll find many others.
If you want to learn a lot about the different ways of doing things, and the use of masks (which is VERY effective for selecting hair), try Deke McClelland’s Photoshop Bible for the version you use. It’s about $40 USD, I got both of mine online for about $22 USD. It’s also in our public library.
Hope this helps and let me know if it does.
Tom Ireland