Close to 40% of my work is softgoods. The process that I find works best for me is to do quick napkin sketches (enough for yourself to go by) and go to JoAnne fabric like YO said and fashion something up. Knowing how to sew is a big plus but you can use needles to stitch things together temporarily. You design it by wearing it (and on other body types) and less about how it looks/materials at this point.