If you’ve never found a good way to get an accurate mitered corner in a quilt border, try this method and see if it works as well for you as it does for me.
1. Cut the border fabric long enough so that the short end of the border fabric goes at least as far as the border on each adjacent side will be. So if your border will finish at 7 1/2″ wide, the border fabric needs to be 15″ wider than the unfinished quilt top.
2. Start by stitching the border onto the quilt, matching the centers and stopping 1/4″ from the raw edge of the quilt. Backstitch. Make sure the stitching just meets at the mark 1/4″ from the corners.
3. Press the seams toward the borders.
4. At one corner, fold the border down so it lays on the inside of the quilt and the short end butts up against, or goes beyond the border on the adjacent side.

Fold one border onto the quilt and adjacent border
5. Find the corner where the 2 border pieces meet on the outside of the quilt and pin right into the corner.

Pin right at the corner where the borders cross on the outside edge
6. Fold back the top border between the pin and the place where the stitching lines meet at the corner of the quilt. Usea ruler to check the angle. It should be 45 degrees. With a striped fabric like this one, you can also line up the stripes so they are straight. I can barely see the fold in this photo because the lines match so well. It shows a little on the far right. Press to crease the miter.

Fold back at 45 degree angle, use a ruler to insure accuracy. Press.
7. Now that you have a crease, lift the border piece that is on top and smooth it out right side up. Align the crease so that it forms a 90 degree angle when placed on top of the adjacent border.

square mitered corner, check with a ruler
8. Now, mark where the corners join and carefully lift up that crease. Use Elmer’s water soluble School Glue or a water soluble glue stick that you are confident will wash out and apply some glue to the underside of the crease. I use Elmer’s School Glue because I know it will wash out.

Glue underside of crease wieh Elmer's School Glue
9. Press crease in place.

Press, making sure to keep a square corner
10. Okay, now that the miter is in place and can’t go anywhere, you’re just about set. Turn it to the back side and you will see the crease.

Crease from backside
11. Pull the quilt center away from the border. It will fold up at the same angle as the miter. You don’t want it in your way when you’re stitching. Notice that the crease starts at the place where the stitching meets in the corners of the quilt.

Fold away quilt center
12. Stitch from the place where the stitching meets in the corners of the quilt to the outside of the border right on the crease.

Stitch along crease
13. Trim away excess fabric.

Trim excess fabric
So there you have it! This is the easiest, most accurate method I’ve used for mitering corners. And since the glue comes out when the quilt is washed, stiffness is not a problem. I love that I don’t have to mess with pins OR slippery fabric!