Gobs of tar. Or anything else that will hold very good. Just line your sideskirts and doorcaps up, mark with a pencil on doors and back of doorcaps where tar or stickyness will go, gob and go for it. Thats how I did mine. But you need to be careful, because if you dont gob evenly, your doorcaps will get wavy.
FYI - Dont plan on removing the doorcaps very eaisily if you use this method.