May be “chicken or egg?” question, some folks prefer the skin first then door bottom. There are some excellent resources on YouTube like Gibson 63 and also Dave Welch from Brothers has a decent door panel series
But I think both say to get a piece of cardboard and trace your door arcs so you have a template to get it to fit the cab
Gibson63:
https://youtu.be/zUp2e6a1wlo
https://youtu.be/DjidWAbsgHA
Dave Welch at BRothers
https://youtu.be/l_RZ6EqVvVU