|When you are sewing something that comes to a corner, such a box side, many instructions to tell you to pivot at the corner. If the fabric is bulky or it is a small piece, making the corner and sewing down the next side can end up messy.
My suggestion is treat each side as a separate seam. Sew from corner to corner. As you turn the corner to the next side, make sure to insert the sewing machine needle so that the seam matches and you don't end up with a gap. And, as always, do a little lock stitch to secure the end to the line of stitching.