Step 1: Assemble your supplies & get started.
A cheap foamy mattress pad from a discount store (about $9 for a twin, I bought a queen for $14)
Fabric, staple gun, staples, scissors, loooooong needle (teddy bear making supplies, upholstery needle, something like that)
Upholstery thread or some other really strong thread...fishing line could work, maybe.
Buttons, cotton batting or felt in a similar colour to your fabric ( only if you want covered buttons on your headboard)
Disposable chopsticks or bamboo skewers & pair of pliers
I measured my plywood and decided I wanted the headboard to be a smidge wider than the bed. I cut it into a 70"x33" rectangle.
You can get creative here and do all kinds of swoops and curly-cues in your headboard, just be forewarned, this will be a pain in the butt to staple your fabric around and have it look neat, tidy and non-bunched. Don't say I didn't warn you! A better idea, I think, is to splash out on some spendy fabric. You only need a couple of metres, so you can splurge on something really grand. The stripe I chose has a kind of pearly, organic, nature-y feel to it.
Decide now if you want to put buttons in your headboard. You can add them later, it's just easier to drill before adding the foam. I found the very centre and drilled a hole. I then went out to each side, 8", then made a diamond pattern, adding another row above and below the centre row. More on this later...
Lay your plywood out onto your foam. You may need to trim excess off in one spot and use that piece in another spot. You can see in the second image (at the bottom, right side) where my seam is. I didn't worry about glueing it or stapling it on the front. Overlapping it would leave a visible ripple through the fabric, so I simply butted the new piece up beside the other one. This worked well in the end and you'd never know I had to cut & paste my foam!
The staple job can be somewhat cursory at this point. You want to do a neat job when stapling on your fabric, but at this point, you just want the foam to stay put. No need to overdo it.