Repairing a sagging car headliner is a fairly simple DIY process that can be done in four easy steps. The task requires a screwdriver, steel brush, foam-backed fabric, box cutter and upholstery adhesive.
Use a screwdriver to remove all the bolted ceiling devices that hold up the headliner board. These include the car's coat hangers, sun visors and dome light, among other things. Take out the headliner once all the bolts have been removed. Leave both front seats in the down position for maximum moving space.
Peel all the old material from the headliner board. Use the steel brush to scrape off the liner's old adhesive and foam. Be careful not to ruin the board during this step.
Cut the new foam-backed fabric in the outline of the board with the box cutter. Make the fabric slightly larger than the board to accommodate the folds.
Spray upholstery adhesive evenly on the board, and quickly lay the cut-out fabric on top of it. Be precise about gluing the fabric down, and smooth out any air bubbles to ensure a wrinkle-free adhesion. Glue the folds as well. Let the board dry for at least two hours. Once the glue has dried, reinstall the board in your car.