I installed headliner yesterday. It went easier than I thought it would. I dry fit one side, trimmed with scissors, 1/2″ routered (3/8 would have been ideal) edge. I glued that side in place with can of contact cement applied with roller, and chip brush in corners, then went to the other side.
I started with middle and worked out and down. Pieces can be trimmed a little on the large size as they can be wedge in easily. I had newspaper on hand to place pieces during final attachment but didn’t need them.
Only one gap, and it was the very first pieces I attached, and pretty small. I may fill in, or not. Took less than 4 hours and more than a quart of glue.