A More In-Depth Airbrushing Tool
The above technique is a simple and easy to use method to smooth skin in Photoshop but there is another method that is harder and involves more work but might give you more control over the touchup effects.
Step One: Open your photograph you want to retouch into Photoshop (File>Open).
Step Two: Use the Lasso Tool to cut out the individual from the photograph to reduce the size of the file in order to speed up the process. Once this is done, Duplicate the Lassoed image into its own layer. To do this click (CTRL-J) and select Duplicate. Do this one more time, so you will now have the original and two duplicated layers of the individual in the photograph. Name the upper layer High Pass and the lower layer Low Pass.
Step Three: Click on the High Pass EYE to hide the layer. Choose the Low Pass Layer to work with first. In this layer, use the Surface Blur as shown in the Easy Method. If your photograph is of high quality, increase the Radius. Lower the threshold until the image gets blurry but is still recognizable. Then adjust the Radius to make the skin look smooth.
Step Four: Click the High Pass EYE to make it visible again. Change the layer mode to Linear Light in the dialogue box. The bumps on skin that you want to smooth out are most visible on the neutral skin tones and not on the darker skin tones. To combat this, add a layer mask (Layer>Layer Mask>Reveal All). Now, use the Apply Image to add a copy of the image in the layer mask itself. This reduces the effects in dark areas.
Step Five: Now, for the hard part. Click on the IMAGE in the high pass (there is a layer in there too but just select the image thumbnail). Zoom in to 100-percent to make this as easy as possible. Select (Filter>Other>High Pass) and start to adjust the Radiant one step at a time until the skin looks smooth.
Step Six: When you are satisfied with this look, click on the layer mask thumbnail and choose (Image>Adjustments>Brightness/Contrast). Increase the contrast and adjust the brightness to make the dark area’s bumps less visible. When you finish with this step, the skin is completely smoothed out.
Step Seven: Unfortunately, all the image is smoothed out as well. Select both the top two layers (High Pass and Low Pass) and click (CTRL-G) to open the Layers Menu. Select Group Layers to group these two together and then add a Layer Mask to the group (Layers>Layer Mask>Hide All).
Step Eight: The image now looks like it originally did. Add a new Layer below the Group Layer but above the rest and make it RED. Change the opacity of the layer to around 50-percent. Now, choose the Group Layer again and choose the paint brush. Start painting over the areas of skin that were smoothed out in your work. As you paint over them, you erase that portion of the layer and the red from the layer beneath shows through. This does not have to be perfect because the smooth effects will not cause more than minor flaws and are unnoticeable. Paint in all the skin but not the hair, clothes, eyes or background. When you are finished, delete the Red Layer.