How do I make a California King headboard and then cover it?

Here is a project on how to tuft. You would need to draw out the shape you want on the wood and cut it then add the foam and fabric.

https://www.hometalk.com/diy/bedroom/furniture/tufting-a-headboard-the-easy-way-4433922

This is a tutorial you could try: https://www.hometalk.com/diy/bedroom/furniture/diy-upholstered-headboard-28904829

DIY Upholstered Headboard

Use a 1/2 inch thick piece of plywood cut to the shape you want. Purchase some fabric glue spray and then spray on the surface and then stick your batting to that so that it cannot move. Go over the edges too and allow at least 3 inches to go over the edges and to the back. Put your fabric over the whole thing making sure you do not skimp as you can cut off any excess you do not need once you have wrapped the fabric over the top and around the edges. This fabric can be stapled on so be sure to pull it tight. On the back, you can purchase a nice clean piece of white cotton to cover the whole back. Fold in the edges and staple that to the back.

Make a templet for one-half of the profile using a roll of brown paper. Using a sheet of plywood, trace and cut the shape you want. At this point I would attach half of a French cleat to the back of the board. You can buy a long cleat from Home Depot or Lowes or, if you have the skills, cut one yourself. Use spray adhesive to hold the foam to the front of the plywood. An electric knife is very useful in cutting foam. I suggest you cut the foam to the profile after it is glued to the plywood. To achive a smooth edge, cover the foam with polyester batting and staple it in place. Depending on your finish fabric you can either cover the batting with your finish fabric or first attach a neutral colored light cotton.