Avoiding sags, gaps and gathers. My experience with application of the liner material to a glue-moistened headliner board. I used the stuff you paint on with a brush, buy about a quart of it. (learned that on the internet, that the sparys are convenient but don't work as well). Made by many manufacturers, carried in auto part shops. I scraped all that loose foam stuff off and sanded it smooth. The last 3% in a few patches wasn't worth removing. The old foam is miserable gummy sticky goey stuff. I did a layup dry run and discovered that I needed 4 assistants to hold up the corners while I started from the center. Once everything is in place the layup only takes 15 minutes. The assistants are essential, but they don't need to be experts, just reliable. Wives, kids and the neighbors kids, then it's everyone for icecream. Worked out perfectly for me and looks like a factory job. I started from the center and worked my way outward in circles. The purpose of the foam must be for sound deadening. Who would notice a noise change in a Disco? I have no sunroofs so that fitting problem was avoided.