I have posted this before ... I think it was even a tut ... but can't find it now. so here it is again:
Sew each border on stopping ¼” from corner edge. Turn quilt Right Side Up [facing you] on an ironing surface. Put one border edge over the other. Work with this top border to form a miter from the inside corner of the border [where it stopped ¼” from edge] to the outermost edge of the border. The top folded under border ‘tail’ should after turning be right on top of the lower most border ‘tail’. Pin tails that are visible. Press well with Best Press. Then gently pin a bit away from the pressed line on the inner side of the press to keep in alignment. Sew right on the pressing line from that ¼” seam stop to the edge making sure that the quilt body is folded out of the way … or you can sew from the outside in …. It doesn’t matter as long as you have pinned it. Backstitch at the end where the quilt body is. After sewing check for appearance again before trimming.
This is the easiest way to do it. If the borders aren’t perfect a ‘fudge factor ‘ can be worked into the miter and it will still look great.
If you have multiple borders treat them as one unit but matching as you form the miter on the ironing surface.
All rotary rulers have 45° angle lines on them. This is all you need to successfully miter the corners when you are adding borders to a quilt top.
Hope this helps.
ali