I am in the process of designing my first quilt from scratch. Usually I just adapt things from other quilts that I have seen. I am using EQ5. I had a vision in my head of what I wanted and then worked it out in EQ. I'm not sure it's quite there yet...not quite as 'stylized' as my original vision in my mind. It's a challenge for me.
If I did not have EQ, for me, I would probably make up some colored blocks/pieces and arrange them accordingly. I don't know that I'd be overly accurate in the pieces I made up initially. You could always work out the exact measurements/layout on graph paper AFTER getting the visual you want.