I do them successfully but it's the dickens to explain,
Yes, it is extremely important to leave enough fabric for the miter. I actually ran short on one border a long time ago, and now I leave plenty. So good for you!
I sew the border up to the 1/4" mark and generally backstitch so it won't unravel. Then I pull the side border up and align it perfectly with the top border. (The corner of the top will be folded like a triangle. The two borders will lay on top of each other.)
I use a 45 degree ruler and draw a line from the end of the sewing line (the back-stitched end point) to the end of the border. Then I carefully pin across the drawn line.
Starting at the back-stitched point, I sew on the line removing the pins as I go. Before I trim the excess and press the seams, I check my miter.
This method works well for me. Good luck with yours.