What You Need:
1 or 2 packs of upholstery nails, depnding on the headboards size
Lay the batting down on a clean floor and place the headboard face down on top of it. Make sure there is about 5 inches of batting that folds over to the back side of the headboard. Use upholstery nails to nail the batting down at least twice on each side. You won't need many nails because we'll use more after we add the fabric. Once the batting is on, pick up the headboard and lay the fabric printed side down on the floor and then the headboard face down on the fabric. Once again make sure there is enough fabric on the back that you can nail it down.
Here's where it starts getting tricky. You'll want to make sure the fabric is taut so there aren't any wrinkles. Start by pulling nailing the fabric on the middle top, then nail the fabric down about 5 inches to the left and right of the middle.
Follow the same technique from above for the bottom and edges of the headboard. Make sure you pull the fabric tight before nailing the fabric down. Once the edges are all pretty secure pick up the headboard and see what parts need to be pulled back more and nailed again. More nails the better!