How do you do it? You can scan tons of forums for ideas, and look at new and existing cases till you're blue in the face. Or you can sit there with your eyes closed and think up stuff......both methods work.
Me.....I like to strip a case bare, and look at it for a long time, from all the angles, over the course of a week or so. I know about what size I'm looking for, and a few other things, but there's still a way to go. In the photo I've started construction of a mid-sized project. The lower filter/intake housing is finished.....and I'm now considering the next steps.