As with just about every film, fans were working on costumes long before the release of Rogue One. They used images from trailers, key art, and costume displays for reference. Over at The Replica Props Forum (RPF), user TheSeekerOfTru7h started a thread about his replica of Captain Cassian Andor’s costume and others jumped into share their progress. The thread is an excellent resource for reference pictures and techniques to craft replicas as well as suggestions for premade boots and jackets to start with for modification.
User WilhelmScreamer replied with his observation about the costume pieces and the patterns he used to make the jacket and shirt. The above and below photos are his.