Recently, I decided to miter the corners on a quilt border. The motif in the material would look better than if it got chopped off at the edge. I could have done a better job of matching up the pattern at the corners since I had lots of extra material to play with. But I shouldn't be pointing out what I don't like, so pretend that you didn't just read this!
Here is one method that I think is tried and true. It involves Elmer's school glue. I think that I learned it first from Sharon Schamber.
When you sew the borders onto the quilt top, start and stop 1/4" from the edge, and do a couple of back-stitches. Press the seam towards the border. Lay the two borders on top of each other, and using a large ruler, square up how the two borders lay on top of each other.
Trace the edges of the ruler with a chalk pencil. I used a white chalk on this dark fabric. I moved the ruler so that you could see this thin chalk line below.
Then place the ruler on the diagonal, going from inside corner to outside corner, and trace that line with the chalk pencil. Again, I moved the ruler so that you could see the line.
Fold the top fabric back on this penciled line and press it. It helps to line up the border edges sticking out on the bottom. The bottom right crease should fall exactly on the penciled line.
Using a very fine tip, place a light line of glue just underneath the fold line. Press. I purchased the fine tip from Sharon Schamber, but I don't know when you'll be reading this blog, so I can't say for sure she still sells them. You may be able to get them from someone else.
After this cools and is clearly glued in place, open up the fabric so that you can see the crease line. Trace the line with another pencil line if you can't clearly see the crease mark. The glue will hold the pieces together in just the right spot.
Pin in place, so that the layers don't shift.
Sew right on the line, back-stitching at the start and at the end.
Trim the seam to 1/4" and press.
I hope you try to miter the corners of your borders this way too. The glue really does help you get a nice result.