I smooth skin with two tools: the blur (I use it at 100%) and the smudge tool (30-50%). You can also just use one of them, but I personally like to use both of them.
Photoshop:
Step 1: open a picture Step 2: go with the smudge tool over the skin. Be careful that you don't accidentally ruin the eyebrows, lips, nose & eyes. Step 3: copy (control + j) the layer & go with the blur skin repeatedly over the skin until you're happy with the end result. make sure that you don't blur the eyebrows, lips etc.
Videotutorial:
Picsart: Step 1: open a picture
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Step 3: go to beautify Step 4: go to smooth
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Step 5: go over the skin & if needed change the size of your brush
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Step 6: when your finished press the thingie in the right corner