We will start with the following image:
We are going to use the Heal tool () to clear the skin in this photo.
We will use a brush Diameter of 15px and a Hardness of 0%, and zoom in to the problem areas. Once we are zoomed in, we an move around by holding Spacebar and clicking and dragging. The way we use the Heal tool is to first hold Alt and click to choose a source. Then we can paint as normal to paint using the source we set. This means that we need a source that matches the skintone where we are going to heal. So we hold Alt and choose a source:
Then we paint on the problem area. You will notice that it might be a little too dark at first:
But when we stop painting, PhotoShop will adjust it to blend it in as best it can:
We can repeat this process for all the blemishes in the photo. It is best to keep the brush smaller rather than large to cover smaller areas. This way we keep the detail of the image, but we need to make sure we select the correct sources for the Heal brush. After healing, we have something like this:
When we are done, if we find that there is too much color variation on the healed areas, and it isn't blended very well, we can use the Spot Healing tool to rectify this. Simply choose this tool, select a similar diameter and hardness to what you've been working with and paint over the areas that aren't blended in so well. This tool does not need a source. It will attempt to heal the area using pixels surrounding it.
After using the Spot Healing brush, we have this: