So, when I completed this project, I didn't add any interfacing fabric or foam batting on top of the upholstery foam. I just went straight to the linen fabric. And for cost saving purposes, I think that was a good choice. BUT for design purposes, it wasn't exactly the best choice. Because the linen fabric was kind of thin and when the headboard was finished, the whole thing had a green tint to it. You could see the foam color through the fabric. It just changed the color visually though, you can't actually see the foam though the fabric.
That said, you have two choices for the next step. If you have a thick fabric that when laid over the foam does not change color, you're ready to cover the headboard in fabric and be done within project. BUT if your fabric is thin and/or you can see the color coming through the fabric, you'll either need to add an under layer of white fabric OR a layer of interfacing fabric / batting fabric.
Once you have that part figured out, it's time to complete the headboard by stapling the fabric (or fabrics) all the way around the headboard. You'll pull the fabric over the top of the headboard and staple it onto the back of the headboard.
I like to start in the center and work m way out. After you have the first staple in, go across to the opposite end, pull the fabric as taught as you can get it and staple the other side of the fabric down. Do that on the sides of the headboard as well, in a few key places, and then you can just go around the whole headboard, pulling and stapling the fabric every inch or two. This part takes a while!