ok, were going from THIS image, to this:
STEP 1) First of all, crop and resize. If you don't know were the crop tool is, its under the dropper tool (5th one down). Sharpen (Adjust>Sharpness>sharpen) it some. If the image is alittle too light, try contrasting (adjust>brightness and contrast>brightness and contrast). Here's what I'm starting with now:
STEP 2) Duplicate this image and then make a new later over it. Zoom in some and look for a good spot for a gray color, im ganna use #ababab. fill the new layer with that (the blank one) and set to screen, merge the screen image to Copy of background.
STEP 3) Set the new copy of background to Difference or Exclusion. Excusion will make it brighter. I'm going to use Exclusion.
STEP 4) Make a new layer and open the gradient pallent: pick any gradeint that describes the mood of the icon. Im choosing Rainbow Pastel. Fill the new layer with this gradient and set it to Color (Legacy).
And you done! If you want, you can add text, brushes, whatever you want! Here a few icon i made using this same style: