JoAnne fabrics for headliner material. A couple of yards is enough (to do it twice when you mess up the first time!).
Remove old material.
Unless the old headboard is completely falling apart, reinforce the BACK, that is the side facing up when installed, not the side that goes to the inside of the car, with a couple of layers of fiberglass mat and resin. OLnly doing the back means you have a nice soft face to glue the material to. If you reinforce the other side, every bump and mat joint will show through the material once it's glued in position.
I've used spray can stuff in the past and it's been fine but it was something like 3m super duper really REALLY strong stuff (forget the exact name!).
Lay material on the headliner board then fold back in half, apply glue to the headline half and material, when tacky, fold it over with a rolling motion to avoid getting creases in it.
When done, flod the other half back, repeat.
Leave plenty of overlap around the edges.
If you have a sunroof, when cutting the opening leave LOTS of overlap. It's surprising how much it takes to wrap into the opening. Better too much than 1/4 inch too little and have to redo it all.
Best way to remove glued on headliner material that I have found is to use an angle grinder with a flap sanding disk to 'GENTLY' grind it all off.
Anything I might say is probably worth what you paid for it, so treat it accordingly!