How to make it?
Measure the pillow you're going to cover. Use a soft measuring tape and measure the length and width of the pillow. Do NOT pull tight we need to full robust measurements, not the suck in your gut size.
Length: ___________ Width: ____________
Cut a piece of batting the width of the pillow and 2 1/2" times the length.
Cut a piece of holiday fabric 1" wider than the pillow and 3 times the length.
Center the fabric, right side up, on the batting. The fabric will extend beyond the edges of the batting, fold the fabric over the edge of the fabric. Top stitch along the fold and the raw edge of the fabric (through the batting). Repeat for both ends.
Find the center of the fabric/batting strip and mark the size of the pillow on the fabric (water soluble or air erasable marking tool is best), centered on the strip. Fold the fabric/batting on the marked lines bringing the right sides together.
Sew the side seams catching all of the raw edges and batting in the seams. Use a 1/2" seam allowance. Once the seam is complete use a zig zag or overcasting stitch to sew the raw edges together and compress them to reduce the seam bulk.
Turn right side out through the opening and remove any markings from the fabric. Add a button and buttonhole if desired. The batting will give thickness to the case, put your pillow inside and start on the next one. If you have a couple of pillow forms you can make a selection of seasonal cases that you can swap out with the rest of your holiday decorations.
Alternatives
You can add an applique, button or other embellishment to the front of the pillow before folding it and sewing the side seams.
Use a couple of "frog" closures for a decorative effect instead of a button.
If you want an invisible closure, add Velcro to the folded edges of the fabric that make up the flaps. Sew one side to the inside of one flap (this will be the exterior flap) and line-up and sew the other half to the other flap. As long as the exterior stitch lines do not go through all layers the stitching and Velcro will not be visible when you fold over the end and finish the flaps.
How about placing a tassel into the four corners before sewing up the side seams.
Have fun. Find the perfect holiday fabric and whip up a few pillows. Remember those great panels you bought a while back and never used. Well here is your chance to show them off!