PROGRAM Photoshop CS.
ORIGINAL Image (Pixiv -artist unknown).
STEP ONE: CROP
Crop your image to your liking. I generally like to keep the original size of the image when doing the basic editing that doesn't involve pre-existing 100x100 pixel textures because it's easier for me to view the finer details (I have a large screen).
STEP TWO: SELECTIVE COLOR
I like my icons to be on the vibrant side so I usually do a bit of selective coloring to bring out some brighter colors. Plus, this icon was entered in an icon contest where the theme was "Purple," so I wanted to amp up the purple hues.
Go to Selective Coloring and make sure you're in the Reds. Make sure Cyan and Magenta are at 100% while Yellow is at 0%.
Okay, now from there, select Blues. Once again, Cyan and Magenta are 100% while Yellow is 0%. Now the purples in Sheryl's hair are more vibrant! If her hair seems too harsh, don't worry, we'll need it for the steps ahead...
STEP THREE: SOMETHING EXTRA
Now that I've got the image looking just about right, there's something missing. While the focus was meant to be on Sheryl's hair, I felt too much attention was lost at her lips because her lip color in the image originally is a pale lavender, but I didn't want the entire focus of the icon to be on Sheryl's hair. My solution? Give Sheryl some color!
I simply Made a New Layer, setting it to Linear Burn at 100%, took a 9 px rounded feathered brush (Hardness should be at 0% so it's more natural and less harsh) in #FF0000 and colored her lips in. To touch up, just erase whatever falls outside the lip line with a feathered eraser. Don't forget to fade and taper the lines as it reaches the corners of her mouth. You can Merge Down this layer to the image at this point.
STEP FOUR: FINAL STUFF
We're almost done. Make a New Layer and fill it with #472608, then set it to Exclusion at 100%. Looks a bit weird, right? Well, there's a solution to that, too.
Duplicate your original image, and move it on top of your Exclusion layer (Original Image ➠ Exclusion Layer ➠ Original Image copy) and set it to Soft Light at 100%. This way, the image comes out more without losing the "tint" of the Exclusion layer.
STEP FIVE: FINALLY
At this point, I resize my image down to 100x100 pixels.
I took the texture above (it's been rotated 90° CCW for the sake of this icon), which I confess I lost of track of who exactly made the texture set it's from (so, if you know or you are the maker, by all means please let me know so I may give proper credit), and set it to Screen at 100%. And you're finished!
I hope the tutorial was easy to follow and understand. If there's anything you're confused with or need more clarification with, feel free to ask. :)
THINGS TO REMEMBER ABOUT THIS TUTORIAL:
As with many icon tutorials, this tutorial is specifically tailored to this image.
★ Using the same Selective Coloring specifications in this tutorial on another image may give you a gruesome result. I adjust Selective Coloring depending on which colors I'd like to bring out, so you'll most likely have to adjust the specs if you apply this tutorial to another image.
★ Likewise, the particular color used in the Exclusion layer may not work for all images. Other good colors to use for Exclusion are dark colors, especially blues, greens, and reds.
Well, that's that, I think. Happy iconing!