Thanks! My only tips for you would be to try to stick with a consistent theme throughout (I guess mine would be monochrome, small squares with rounded corners) to create a unified look. If they're going to have that low-resolution look, you definitely want to keep the complexity down, because you can only fit so much detail in a small icon. This probably means you'll have to stick with symbolic, easily recognizable images for your icons. Of course, it's always good to add a bit of originality to your design, whatever that may be.
Haha, well I actually used GIMP to make these myself. As far as resolution goes, you'll want to actually make the icons in the smallest scale possible, where what looks like 1 pixel is actually 1 pixel and have the canvas size probably be some common icon size (like 24x24, 48x48, 64x64, ...). From there, you can scale the icon up to a certain size, but make sure you do it with a percentage divisible by 100 (200%, 300%, 400%, ...), or the pixels won't all be squares. Also, make sure the interpolation is set to "none" or else it will smooth the image out, which you don't want.
Since you should make the icons at 1 pixel scale, the best brush to use would probably just be a 1 pixel square. If you're creating a common visual element that is present in every icon (like a box, border, or some other embellishment), then it might be a good idea to make a template first and then use that to make your icons. Glad i could help!