How do you achieve this skin texture effect on Photoshop?

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Pretty much everything done in the actual effect is just careful adjustments (using adjustment layers: Layer > New Adjustment Layer > …) to slightly alter colors, contrast, and such. Masking is also heavily used to limit where the adjustments are made. It is also possible that some overpainting was done in certain areas to make some parts of the photo look much smoother than in the original.

The tutorial was grabbed from this site:

http://learnphotoediting.net/

which is more straightforward about asking you for a purchase, and likely goes into deeper detail regarding the process used. If you're curious about the technique and want to support the creator, use this site instead.

With some thoughtful searching, you can turn up more useful tutorials on other sites, such as deviantART. And there is nothing stopping you from just experimenting with all of the tools that Photoshop offers; just remember to save copies and keep your originals safe somewhere.