I’ve done sofa cushions. If making self piping make sure the piping cord is very loose so it doesn’t twist and so it bends easily, inside the tube, double stitch the corners with small stitches and cut the corners so they bend

are you making your own bias binding? Make the binding at least 5 cm wide

if buying sew in cord same goes, have it loose so it doesn’t twist or bend, double sew corners and snip the corners