When I started quilting 20 years ago I couldn't afford quilting books and certainly didn't have any internet access to look at how other people did things. I had read how to miter somewhere that it said to fold the quilt at the corner in a 45 degree angle, draw a line out to the edge of the border pieces, and sew on that line. That didn't always come out right for me and I was dissatisfied with the results. What I wanted was that knife-edge straight line I got when I pressed the miter so I came up with a way to keep that nice edge and alignment.

I don't know if anyone else does this and I haven't seen it anywhere so, here goes with a tutorial on it.

When I am mitering a corner I still measure the quilt through the middle and instead of cutting the border that length (you need more length past the ends for the miter), I use a fabric marking pen and mark the quilt edge line on the border. If your border is 6" wide then you need to have at least 6" past the edge of the quilt. I add at least 1" past the needed length.

Sew the first two opposite sides of the border on as usual. Mark and sew the second two opposite sides the same way and be sure to start and stop the stitching exactly at the edge of the quilt, don't sew into the opposing border at the corners.

Lay the top on a pressing board, making sure it is square, and carefully press the borders.

Lay the second border over the first making sure it is square and flat.

Fold under the top border strip so that it makes a 45 degree angle on top of the bottom one. If you have stripes or something you can line the up on the inside and the outside edges.

Without moving the piece stick pins along the exposed edges to hold them in place.

Carefully fold the quilt top back exposing more of the borders. Pin again to keep them from moving.

Carefully keep folding the top back until it is flat and the pressed line is exposed.

Smooth the border down with your hands and pin on the other side of the pressed line.

Slip a finger under the edge an up to the first pin. Gently lift the material and the pin, make sure the pin stays in both layers of fabric but comes out of the pressing board. Pin both layers together.

Pin, Pin, Pin. Keep moving up the line and securing each pin. I then pin across the pressed line so that it doesn't move as I sew it.

Sew on the pressed line. I do this slowly and I also leave the pins in, going very slowly over the pins so I don't hit them with the needle.