Theres several ways, you can use software like portrait pro or portrairure or another popular way is to duplicate the layer erase or mask out everything but the skin (erase mouth eyes hair background etc) then add some bluring to that layer and ajust opacity to suit.
Any blur approach is not suitable for serious fashion or beauty at all, as the results look plastic or, well, blurred. You really need to separate low-frequency detail from high-frequency, preferably with intermediate frequencies as well, otherwise you'll destroy pores and the results look plastic. I've found this tutorial
to offer the best compromise between ease of application and good results. Here
a crash-course of a simpler approach.
Many pro retouchers rely on dodge and burn - it's tedious and time-consuming
, but produces superior results.