Once within 1/2 inch (~13 mm) of a patch corner, create a corner stitch:

Insert the needle into the patch at the corner, 1/4 inch (~6 mm) in from both edges.

Then bring the needle out at the corner edge, passing through the thread loop.

This will help the stitching round the corner.

Then continue stitching down the next side of the patch.

It's okay if the interval length of the stitches varies slightly at the corner.