1. Prep Your Base.
I always crop them, resize them, and then sharpen them (Filter>>Sharpen>>Sharpen), because I like a very sharp picture.
But sharpening won't always be necessary depending on your image. Lately, I've also been using the auto corrections (image >>adjustments>>auto levels/contrast/color) to balance out my images before I start working on them.
2. Selective Coloring.
This tutorial is strictly for the coloring style anyway, so this is the part you'll need to pay the closest attention to. You'll need to create a new adjustment layer for selective coloring (layer>>new adjustment layer>>selective coloring).
And you'll need to imput these numbers:
Reds: -100, 100, 100, 100
Yellows: -100, -100, 0, -100
Greens: 100, -100, 100, 0
Cyans: 100, -100, -100, -100
Blues: 100, 100, 0, -100
Magentas: -100, 100, -100, 0
Whites: 100, 0, 0, 0
Neutrals: 30, 0, 0, -30
Blacks: 0, 0, 0, -10
We'll be adding a saturation level (layer>>new adjustment layer>>hue/saturation) next just to give those colors we set up a boost.
Imput the following:
Master: Saturation: 15
And TA-DA! There you are. You can always take your icons to the next level by adding brushes and textures. But I love the basic and clean feel to this, so I left mine pretty much as is. Hope this works well for you, and I would love to see what kind of results you get. ^_^
More icons made with this tutorial: