The trick with the headliner, pierce the can with a large screwdriver spray like an idiot all over the old liner, even better if still fitted to the car, then get the new material and jump all over it , then crumple it up. Then climb inside the car and lay on the back seats, yes in the excess glue and stick it to the roof with your bare feet( added sock fluff). Then proceed to the front and do the same, making sure to leave plenty of gluey' hand prints on most panels.
Then, grab a few live or dead small animals and insects and throw, place and crush them into wherever you feel looks best.
This may seem an extreme technique but makes interesting conversation with the mother in law on the way back from the airport and kids love it damn, dont forget flour! Great things are made from flour!