How do I attach upholstered headboard pieces?

You cover the front of the sides first leaving some excess fabric down the back opening. The sides are then screwed into the side of the back. The excess fabric from the sides is then pulled around the back and stapled to the back. That will hide the screws. The back fabic or covering is then stapled on last to tidy up the back and hide the excess fabric from the sides. Most people dont cover the back. I personally use a black fabric weed mat on the back of the furniture I make.

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You can use 20mm thick board that would work fine. Then you can add an extra 20mm frame about 50mm thick around the sides and top to make them thicker. Then sides can be 2mm thick if it does not stick out to far. If you want it to stick out further add 2 x 20mm boards this will offer more support. When you screw the sides into the side of the back. You would pre drill the holes first to stop the wood from Splitting. And for extra support you can also glue it in place. It is no big deal to see staples because it is against the wall and you will never see it. If you have your mind set on not having any staples visible. This is how you do it. The top section of the back is attached first using upholstery cardboard tack strips under the seam to keep it straight and to stop the fabric from pulling down. The sides are then secured to the back using metal tacking strips. They get wrapped in the fabric and hammered into the wood. Lastly the bottom is stapled underneath. Hope this makes sense.

You use a staple gun. Remember to do each middle first. Then each side of this, do corners last. If you have downward posts then unscrew before upholstering then reattach after.