I've mentioned it before but cushion covers are not my favorite part about upholstery work. Why? Mine used to come out a little tweaked. The top and bottom never seemed to line up perfectly in the corners so I was constantly pulling them apart and redoing them. I have an old sewing machine with no bells and whistles. The problem I was having is even with tons of pins the fabric still pulls and stretches and by the time I got to the end the pieces were no longer lined up. It wasn't until I heard this tip that I started getting the results I wanted.
Tip: use staples instead of pins.
Start by sewing the piping to the top and bottom pieces of your cushion cover.
Then sew the middle (boxing band) to the top piece using staples to hold the fabrics together. Space the staples about two inches apart. Staple in the middle of your seam allowance not all the way up to the piping or else you'll break a needle.
When you are done sewing use a staple puller to easily remove the staples. Mine is on the back of my stapler.