increase the skin colour range to maximum to get as much skin as possible included - this will include non-skin but that will be excluded via a mask:
On the Mask menu at top of program, choose View Mode and select Red Overlay and turn on Mask Preview (O or icon at bottom of screen) to make it easier to see where you will paint in on the skin (the red area will be the parts NOT included in the mask)
click on masking icon to left of layer title
click on Invert so you can then paint in the effect on the skin
masking brush on far left should already be selected
ensure the settings at top of screen are Paint In, set feather to around 50, adjust size of brush
consider using the Perfect Brush, especially if subject has darker coloured hair
paint over the skin you wish to be included, does not need to be super accurate