If I was stranded on a desert island and had to choose just a few Photoshop tools to bring along with me (assuming this desert island had laptop computers and digital cameras), the cloning stamp tool would definitely be on the short list. As mystical and magical as I find almost everything about digital-image editing, I am most enchanted by the ability to "clone" pixels from one part of an image (or an entirely different image) to another. Essentially what the clone stamp does is replicate, exactly, the pixels you tell it to copy and then places them wherever you want them. Just how cool is that?
There are a lot of practical applications for using a clone stamp and the more you use the tool, the more uses you'll find and the more your admiration of this profoundly helpful tool will grow. One of the most common uses for cloning is to "erase" things that you don't want in a shot. If, for example, you've taken a great and very pristine-looking shot of a saguaro cactus in Tucson only to find on closer examination that you've inadvertently included someone's discarded coffee cup, you can literally erase it from the photograph and no one will ever know it was there (and don't you wish we could clean up the planet as easily).
You can also use the clone to erase larger things that you no longer want in your photos--like taking dear old Auntie Maude, who wrote you out of the big will at the last minute, out of your holiday photos. She never smiled anyway, the old hag, banish her. In the photo here, for instance, I was able to completely eliminate the treasure hunter from this Florida beach. Not that I have anything against my fellow treasure hunters, but it makes a good demonstration point. And just so you don't think that I waited until he left the scene and shot a second picture, when I cloned him out, I carefully left his shadow. I repeat: How cool is that?
Using the clone stamp is a lot easier (and even more fun) than you might think. If you were sitting here next to me, I could have you erasing select relatives in a matter of moments. Basically what you do is decide which part of the image you want to hide and what you want to hide it with. You then "sample" the replacement material (again, you're just copying pixels from one area to another) by clicking on that area (Option-click on a Mac, Alt-click on a PC) and then use the cursor to paint those pixels over the area that you're covering.
The only real choices you'll have to make are what size brush to use when cloning and what level of hardness to use. It takes some experience to know which size brushes work best with certain subjects and what level of hardness makes the cloned areas most invisible, but you will quickly see what works and what doesn't. I suggest always working with the image at around 100% enlargement so that you can work in greater detail but with a larger image, that way when you shrink it back to it's printing size you are far less likely to spot any minor flaws. Also, work slowly and with a smaller brush; this takes more patience and makes the work go more slowly, but the results are usually more precise.
There is more to the cloning technique, but most of it you will learn through trial and error. For example, when I erased the treasure hunter I continually resampled my source area and took that source material from immediately to his left or right. By doing this you keep the same level of sharpness in the replacement material as you had there before. If your sourced material from a different lateral area you might be in a different sharpness zone (in terms of depth of field, for example), so you'd be cloning material that was more or less sharp than the area you were replacing and that would show. Once I had the man erased, I then looked carefully at where he'd been and cloned in some bicycle tire marks, stones and footprints to make the beach where he "had been" look like the remainder of the beach around him. I know, it's magic, it really is--but it relies on your skills too.
By the way, yes, you can clone from one photo to another! (Need I say a third time: How cool is that?) Just have both images open, sample from one and deposit in the other. I've taken flowers from one garden scene and placed them in another and it takes just seconds. You could, quite easily (and gleefully), place old Auntie Maude's head on the body of a wild goat.
OK, one last time, all together now: How cool is that?