There are many ways of applying selective
sharpening to an image. If you applied it
to an image in the conventional darkroom,
the process will be familiar.

1- Open the image
2- Layer>Duplicate Layer...>Background copy
3- Filter>Other>High Pass...>Radius 1.0 pixels

A silvery image will appear with a very faint outline of your subject
4- Change the blending mode from 'normal' to 'Overlay'

"After step 4 you can select which areas you want sharpened by adding
a Vector Mask and using Ctrl+Delete to change it to black. Now paint
the areas you want to have sharpened with white and they will show up,
while the other, unpainted areas, remain hidden."- Fatheroftwo

You will now have sharpened the image the same way as you
would have in the conventional darkroom way with a proper 'unsharp mask'

By changing the blending mode back to 'normal' you can use the erasor to
remove any areas that you do not wish to be sharpened up by erasing the grey
sections. The adjustments are visible as embossed ridges. You can also
select how much you wnat the sharpening to be applied by adjusting the
'opacity' which is set at 100% by default..

Yet again, there are more than 100 ways to achieve the same or similar
results, but this is a simple means that I tend to use. It may not be the
best way but I like it, and therefore share it with anyone who wishes to
try it for themselves.