Here is the picture I am starting with:
There are several things that I would like to do with this. First, she has a noticeable bit of drool on her chin. She also has a slight rash under her chin and a few small scratches. The birthmark on her temple also grabs attention more than I would like, though I don't want to get rid of that completely because it is more permanent.
Before making any changes, I started by making a copy of the image, called a Background copy (see here). This is basically a layer over the image, and if you mess up, you can delete the Background copy without any harm to the original image.
Make sure you have the copy selected, then pick a healing brush. Note that if you click and hold down on the button, you will have several options. The Spot Healing Brush will choose what it considers the best option to correct your image; that does not mean that it IS the best. I prefer the Healing Brush tool. This tool lets you pick what you want to replace the blemish with. You hover your mouse over a good area, press the Alt key, then click. This is best when it comes to skin tone. You can select skin right next to what you need to change. You may need to experiment a bit to get the hang of it. Try to pick blemishes in easy areas (skin all the same tone) to practice. No matter what color you select, be careful of getting too close to noticeable darker areas because a darker tone will be blended in. As for brush size, you want it to be just larger than the blemish.
A crucial and easy (but not obvious) thing to know: if you mess up, find the history button. It is easy to get enthusiastic clicking over large areas. When I was working on the rash under Kira's chin, I got a bit too close to the shadows a couple of times. "Undo" would have only fixed one of those. On a related note, save often. Trust me. :)
Once you know how to do that, minimizing things like birthmarks is easier than you might expect. First, drag the layer called "Background copy" to the icon you used before and make another copy. Why another copy? You will need to make what you do partially see-through, so if you do this on the same layer, you'll be able to see through all of your other changes, too. On your new layer, use the Healing Brush tool to hide the birthmark. Then adjust the Opacity slider as shown here. When you are finished, you can press Ctrl+Shift+E to merge all layers together so that you can save the file as a .jpg. Just make sure you're happy with the changes first!
My final picture looks like this:
Notice how even though her birthmark is still there, it is no longer quite so distracting?
All of the changes were easy to do, at least once I practiced a little with the Healing Brush. Have fun!