I started by sketching out some ideas for icons, based on a square grid, so that all icons would fit in the same space, regardless of aspect ratio.
Refinement and review
I brought the sketches into Illustrator to begin working through some ideas. It's a messy process at this point, but I don't like to limit the ideas by worrying too much about the grid, pixel perfection, or layer naming. This is a rough canvas.
Building a cohesive set
Now is the time to put the grids to use. I start by building the icon at the largest size in the UI, 64px. Creating lines at a weight that will scale down (and up) well, without loss of detail. By using the green guides, I am able to see what the aspect ratio of the icon should be, in order to fit into the full set. If creating a horizontal icon, it should fit into the same bounds, rotated 90º.