It should be a thin and hard compressed foam material.
I used 3M spray adhesive on my Mustang and a host of other cars I used to own.
Clean up the board first.
Use a stiff brush to remove the loose material.
Stretch the material over the board and see how it fits.
The material should have come with some instructions, but if it did not; wait to trim it until after you have glued the material to the pad.
On all of the headliners I have recovered, I took the material that overlapped and glued and/or pinned it to the back of the board.
I used a bolt of fabric I picked up on the cheap at a department store clearance sale on these vehicles.
I have no doubt that the professionals have better methods and tactics to get this job done.
This is the way of the "Jack of all trades and master of none".