The sweatshirt use is the best idea to get started. Cut waist and arm ribbing off and cut open the side and arm seams. Now lay it out flat. You have sort of a north, south, east and west look. The first pattern I used suggested not to cut the front until it was either staystitched or patchworked over. this was to give stability so that when the center front was cut it wasn't stretched. I have made three sweatshirt jackets and they wash and wear well. First one is really raggedy looking now but that is fashionable (to me) and I love it. Last one was a stripped raggedy flannel jacket where we sewed widish double
flannel strips together using a 1" seam allowance. When pieces were wide enough we cut out the very plain jacket parts. Sewed the pieces together so that the jacket was flannel self-lined and clipped the outside seam allowances. Fun jacket to wear and warm for my Los Angeles, CA weather.