Cut two 13 inch by 19 inch back pieces out of the decor fabric.
This is for the back and so they can overlap.
Laying the sweater front piece with the piping down with right side facing you, place the two pieces of decor fabric over the sweater, right side facing the sweater piece, overlapping the two pieces of fabric. Ideally, you would hem the long edges of the two pieces where they overlap in the back of the pillow. I cut my fabric on the manufacturers edge that is already finished, so I skipped this step.
Pin the decor fabric in place around the piping, right sides together. It should overlap with the edge a bit.
Take the pillow cover over to the machine and sew the cover together around the piping, still using a zipper foot to get as close to the piping as possible. This can be tricky because you can’t really see what you are sewing, and you really have to feel what is going on. I put my hands down around the piping, trying to sew as close to the piping as possible by feeling the piping. Make sure to round those corners so everything comes out looking really nice.
Trim the excess fabric off the edges, leaving about a 1/2 inch. Finish the edges on the inside so there are no raw edges by serging around the edges. If you don’t have a serger you could just sew around the edges using a zigzag stitch.