the corners are the only tricky bit in the whole thing, and that's mainly because the instructions make them so.
get it lined up as I said, and then sew, bottom section on top, from the'midpoint' of the corner [I put pen dots to help] straight across one edge to the next corner.
Fasten off thread, rearrange the fabric, and do the next side. Sew all four as separate seams, instead of trying to turn the corner.
Then sort of adjust any excess at those corners , effectively making darts or tucks along those seams. Cut away any leftovers. The shape will not be harmed.
How do I know? Because I buggered it up on the first one, and had to faff about. This is the best way!