How can I make a fabric headboard for a king sized bed?

Cut plywood, or another wood of your choice to the design you want. Make sure you have enough overlap on each side of the bed. Use batting and pull it over the edges and staple to the back. Cover with fabric. If you like the design, you can nip and tuck it with buttons. YouTube has many videos to help you do this. Get a strong fabric (not satin, silk, see through, etc.) unless you back it with a very strong fabric such as a canvas, etc. Get a hand washable fabric (not machine washable fabric.) A fabric you can wipe off the dust and dirt with water. If you use stripes, don't make them straight vertical or horizontal. Instead, turn the fabric so the stripes are diagonal. Much more interesting. Pull first from one edge, staple, then opposite edge and staple. If you've done top and bottom, go to sides next, or do sides, and then go to top, then bottom to staple fabric. This is at least a two person job. I wouldn't attempt it by one self.

Use plywood for a panel. Attach it to legs that will fasten into the bed frame. Put padding onto the plywood and then stretch fabric over the plywood panel, stapling on the back side.

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Plywood the size you want you need fabric batting put along the whole board then cover with material you want turn over then pull material and staple pull staple keep doing that till all done