If you’ve ever covered a piece of foam over a plywood seat hospital-corner-style, you’ve probably ended up with corners looking something like this. Oh, sure, it looks better than the original fabric, but the gaping fold suggests a DIY job.
Using curved upholstery needles to do a quick slip stitch* to bring the two sides of the fold together creates a much more finished look. Certain fabrics seem to work better at hiding slip stitches than others. I’ve had good luck with chenille–as in this example–and textured micro-fiber suede.
*Slip stitch is done by sliding the threaded needle through the folded edge of the fabric while at the same time picking up a few threads of the fabric opposite the fold. Continue stitching in this manner, making a stitch every 1/8" to 1/4", evenly.