I installed over fifty of them back in the sixties/seventies..
I only used brush on setup if I was at a body shop type on site location doing partial/repairs etc. Because of the consistancy. Brush on is diffucult to spread evenly. Gets lots of "boogers" in it. Your work area has to be somewhat warm to stretch and form the vinyl.
The best way is to use a sprayable cement. I had two favorites that are no longer around. Kind of a green-yellow color. Came in 5-gal containers. Used a LARGE nozzle in a JGA gun with a pressure pot. Cob webbing can be a problem in hot working areas. I would shoot the seams first. let it set up, then carefully fold seam under pressure. The full length. Then shoot top. Shoot roof. wait till dry, then lay out glazed butcher paper on 1/2 the roof. Lay and stretch out out the other side. Then carefully slide the paper out as stretching the next side.. Really takes about six hands..