It really depends on what you want. It can be as simple as taking wood pallets or plywood and covering with foam and then fabric or vinyl. Putting together several artists canvases and cover in fabric or paint and hang. A siple quilt or fabric can be hung or hung like on a rod, Cover plywood or hang on the wall siveral mirrors ie 12 inch by 12 inch - can be done with tape or hanging hooks ie 3m. The list is endless.
I would take an old headboard and modify it as it will already have places to connect to your bed frame.
If it’s wood then you can glue & screw into it, to make it a King from a Double or whatever you want.
I used old shutters to make a king headboard. Connected them together and then to the wall.
Hello, here are some links for your consideration. Hope this helps
Bed headboard design
21 Clever DIY Headboard Ideas to Spruce up Your Bedroom | Hometalk
Door To Farmhouse Headboard DIY