Plan the Grid
The grid is defined by narrow strips of wood attached to the frame, but it may influence the height of the panel you build. Amy Wynn's headboard had a 4-over-4 grid pattern, for a total of 16 individual sections. The grids do not have to be perfectly square, but they should be consistent. The grids on Amy Wynn's headboard are about 16 inches wide by 24 inches high. You may want to sketch out several grid sizes on a sheet of paper, but the best approach would be to use painter's tape to create various configurations on the wall (or the sheet temporarily tacked to the wall).
When planning the grid, give some thought to where your head will rest when sitting up in bed. I would want to avoid having the back of my head resting against a piece of wood while reading in bed.
Plan the Frame
The visible portion of the frame consists very simply of four boards attached at the corners. For additional support, plan to add a couple of horizontal boards inside the frame after the upholstered panel has been installed. These boards will both help hold the panel in place and, if necessary, provide a surface through which fasteners can be driven into wall studs. Also, since you will almost certainly have to use two pieces of plywood, plan to place one of the boards at the joint between plywood panels to provide support.
Amy Wynn built her frame out of standard 2-by-4s, which were painted along with the wood strips in the grid to match the other furniture in the room. You might be happier with the finished project if you use either 5/4-by-4 or 1-by-4 pine boards instead, which will require less sanding and sealing before being painted. If you plan to paint, you might want to use MDF, which you will have to cut into strips, or poplar instead. Oak or maple with a clear finish would also be a suitable choice.
When planning the frame, keep the full thickness of the headboard in mind. If the wood strips used on the grid are 1 and a 1/2 inches thick and you use 1/2-inch plywood and 3/4-inch supports behind the panel, that will leave you about a 3/4-inch space to fill with batting and fabric, which ought to be plenty of room. If you opt for using thicker foam rather than batting to create a more comfy headboard, you may need to adjust the frame size accordingly.