um
Pick a dark color that matches what your trying to shade( eyedropper tool on the color your gonna shade) go to your color picker and just drag it down till its dark enough for you
then paint over the area you want to shade, go to filter, blur, Gaussian blur, and just drag the slider everywhere until u get something that looks reasonable, you can then use blend modes and opacity to get something you like
EDIT: you have gimp, Idk how to gimp, only photoshop, I'm sorry :c ignore everything I just said
also paint sharp edges not blurred edges :c
Last edited by Kirito; Mar 23, 2016 at 12:53 AM.