Measure the distance across your headboard plus the length of both sides. Add a couple extra inches. This is the amount of trim needed for the slipcover.
Purchase the fabric, trim and other supplies. Pre-wash the fabric and trim in cold water.
Lay Fabric A on a table with the right side up. Lay Fabric B on top of Fabric A with the right side down. Make sure the selvages are even. Place the headboard on top of the fabric. Trace around the headboard.
Repeat this process with the batting if you intend to soften the headboard. Staple the batting in place on the headboard.
Cut the two layers of fabric out. Allow at least 1/2 inch for the seam allowance. Allow more than that if the headboard is extra thick or if the batting is extra fluffy.
Pin the trim in place between the two layers of fabric. If you would rather, baste it in place. Pin the fabric layers together.
Stitch through all thicknesses of fabric and trim. Turn headboard slipcover right side out. Make sure you caught all the trim as you intended. Pull the corners out with a straight pin.
Place the slipcover over the headboard. You don't need to hem the bottom edge unless you want to because it's out of sight. Embellish the headboard with ribbons, lace or bows to suit yourself.