Now you could call it a day and your cover would be finished but I didn't like the pointy corners on mine so I sewed across the corners to give the cover a bit of volume.
Here's how I get it nice and square.
Arrange your cover so it's in roughly this shape then stick a pin into the seam and check that it comes out right in the seam at the bottom. Push it back up through the seam again. This should line them up perfectly. If you have a cutting mat with a grid (or graph paper) you can check that it's all square by putting the point on the darker 5cm mark and then making sure the sides cross the 5cm intersections. Umm that's kinda hard to explain, see the notes on the photo.