Start with this. The most basic kind of icon: just take a screencap and crop it.
Next comes basic cleanup. Duplicate layer. Go to Image → Adjustments → Levels. Move the arrows at the bottom so that they line up with the start and end of the graph:
In this case, that means moving the left (black) arrow up to 47. In some cases, you might have to move just the right (white) arrow, or both.
This is what you get:
Better lighting. Duplicate layer again (as a note: except for the last few steps, you don't actually need to do all this layer duplication, but it helps if you mess up. There are other ways to do this, but I'm lazy and this is the way I got used to). Go to Image → Adjustments → Color Balance. You have three options to fiddle with here: shadows, midtones, and highlights. In each of them, you can control the scale of cyan to red, magenta to green, and yellow to blue.
You don't always need to do color balance. Sometimes you can move directly to the next step. I used to use selective coloring instead, but I've found that color balance is more effective. In this case, this picture is mostly light colors and has a lot of cyan, so let's fix that. In the color balance box, go to Highlights. Move the sliders around until you find a balance that looks better than the original.
Now go to the other options--Shadows and Midtones--and do the same thing. You shouldn't need to do as much wiggling here, since like I said, the image is mostly highlights.
Duplicate layer. (Again, you don't need to, but I do it anyway.) Go to Image → Adjustments → Brightness/Contrast. Fiddle with this until you think it looks nice. For most images, especially light ones like this, you shouldn't need to move the sliders too far.
Duplicate layer. Go to Image → Adjustments → Hue/Saturation. Move the saturation slider up until you like what you see. Specifically, it should look just slightly oversaturated at this point--you'll fix that later. Don't go overboard. That way lies pain.
Duplicate layer. This is the first time you actually need to do it. Go to the Layers palette. In the upper left corner there should be a drop-down menu currently set to Normal. Open it up and set it to Soft Light. Go to Image → Adjustments → Desaturate.
If you're satisfied with what you see here, you're done. But I generally have another step to tweak the coloring and lighting just a little more.
Go to Layer → New → Layer. Study the image you have. Does it still lean a little much towards one color? Is it too light or too dark? In this case, it's still somewhat cyan, with a touch of too much magenta. So click on the top color box and select a color. In this case, we're going to use a dark orange-red. Click the Paintbucket on the toolbar. Click on the image to fill the new layer with the color you've picked. Now go to the Layers palette again and, once more, set this layer to soft light. That doesn't look so good, so play with the Opacity setting until you get something you like.
I may do a tutorial for how I make fancier icons later.