Frequency separation can be a very overwhelming technique used most commonly for skin retouching. It is a technique that is very easy to overdo and destroy texture when not used correctly. If you are unfamiliar with this technique, where do you start?
For those of you who are leery about using this technique, this video from photographer Sheldon Evans shows how to actually go through the process of separating the texture from color layers the manual way. There are plenty of actions and plugins out there that can help you perform this action, but if you don't have one already, follow the steps found in this video, and do yourself a favor and create an action from it.
Just remember there isn't a magic number for the blur value that works on all images, you will have to find the perfect value for each image. Also, you can try out other blur filters and see which one works best for you.
After creating the separate layers, Evans goes into showing how he edits out stray hairs and other imperfections in this video. While doing this, the key is to grab good texture areas to replace others. Focus on keeping the changes subtle and preserving the textures. Going too far can turn your subject into a porcelain doll-like creature, unless that is what you are going for.
Hopefully, with Evans' video, you don't feel so overwhelmed and can start using this technique successfully.