1. Crop base image.
2. Levels. I explained how I use levels in these tutorials, so I won't go over the whole thing again, because it's both exactly the same and at the same time different for every icon. Just basically brighten up the base :)
3. Merge-duplicate what you have so far (Ctrl+Shift+Alt+E) and then take that new layer and go Filters > Other > High Pass, set to about 60. Then duplicate that layer. I set the first to Screen at 37% and the second to Soft Light at 46% - just adjust the opacities until it looks good for your particular icon.
4. I took this texture by (I think) chaoticfae and set it to Multiply at 100%.
5. Now I thought it was looking a bit overly dark, so I took the two previous layers (the screen and soft light ones), duplicated both and moved them above the multiply layer, and set both of them to 46% opacity.
6. Next I took this texture by tihana and set it to Screen 100%. I used a layer mask to tone down (but not entirely remove) the bits covering his neck.
7. I thought it was looking a bit too light, so I took this texture by bambinainnero and set it to Darken at 33%.
8. Selective Colour layer. Often when I use a yellowish Multiply layer in an icon, I later want to make it less overwhelmingly yellow, so I make sure to really tone down the yellow with Selective Colour. If you don't do this step, it will just bit a bit more yellowish :)
Yellows: Cyan +15 Magenta +15 Yellow -43 Black +21
Neutrals: Cyan +13 Magenta +11 Yellow +10 Black -13
9. Hue/Saturation layer. I increased RGB Saturation by +18, Reds by +9, and Cyans by +16 to make the colours a bit more intense.
10. I merge-duplicated everything (Ctrl+Shift+Alt+E) and did a little bit of sharpening around his eyes, just with a soft brush at about 10% Strength. Then I added text and decoration - the font for the text is Baby Bowser 14pt, crisp, -25 kerning, in a dark red-brown shade I picked out of the icon, and the decoration is a little brush from boundary, from this set called 'Ardenwood Swirls', in the same shade. Both are set to Hard Light 100%. Ta-da!