Here’s a great introductory retouching tutorial by photographer Sara Kiesling, who writes,
Basic skin retouching using frequency separation and dodging & burning. I use this process on every photo that I do, and I usually spend about 4-5 minutes on headshots like this (and less time on full body shots when there is obviously less detail in the face). This is not intended to be a high-end retouching tutorial, but techniques that can help people who want to do natural-looking retouching while maintaining most of the natural skin texture!
Frequency separation is a technique that allows you to give skin a smooth-yet-sharp look.
Here’s an explanation by retoucher Ben Secret for his tutorial “Retouch images with frequency separation“:
[Frequency separation is] a technique that enables you to selectively process not only different areas of an image, but also different detail levels. Frequency separation involves creating a high detail (high spatial frequency) layer and a low detail layer from a source image […] Using this technique enables you to smooth and rework rough and fine details independently, and opens up some very high-quality and non-destructive methods with which to sharpen your images.
Julia Kuzmenko McKim has a great text-based tutorial as well, titled, “Skin Retouching – Frequency Separation Technique.” Her page also includes a number of before-and-after examples that you can switch between by dragging a slider.
You can find the finished version of the photo in Kiesling’s video here.