Now the fun part!
Like the batting, start stapling the fabric along one side. Then pull taut and staple the other side. For the corners, cut the fabric in slits, cutting away excess fabric as needed.
You can cut away any excess fabric. I chose to staple it so that when I moved the headboard in the future, my fingers wouldn’t touch the particle board.
The headboard looks fine as is. If you chose a patterned fabric, you would be done right now. But I wanted to hammer in nail head trim.