For information on what is Frequency Separation Technique click here
Click on the stamp image button to create a duplicate layer of the image, or select the layer you want to Separate
Click on the measure button. The action will apply a Gaussian Blur filter to the image. Move the pixel amount slider until the details of the skin disappear. The more you blur the less details you will see. Find the correct amount by over blurring the area of skin you want to retouch. (Or all the skin if doing one pass). Remember this number and cancel out of the blur dialog.
Rename the layer with the blur amount you determined using just the number. Eg. 3.6px use 3.6, 15px use 15. And click on the SEPARETE button. The script will separate the image in two layers using the pixel amount specified. If you do not name the layer, it will default to 21px.
That’s it! At this point you are ready to retouch the image on the High and Low layers just created.