Ok with the quilt right side top, fold one of the borders under to
make a 45 degree angle, playing with it til angle is right. Iron.
Then instead of pinning, I had a piece of tape long enough to tape both sides of the miter together, instead of pinning to the ironing board . After taping..
*Flip over the folded 45 that was ironed and use a ruler and pencil and draw a line along pressing crease. *I just gathered up a little extra fabric to flip (right sides together). Sew along
the penciled line. You can feel the tape edge and seam line
should be just outside tape on opposite side and sew just outside the penciled line. Pull tape off and press again. As I had to move (putting right sides together, I just pulled some slack,
and as it is a very large quilt top and a ways to machine, I put a few pins intape to keep it secure.
So this eliminates pinning to ironing board and pinning angle
and there are two guides for exact seam. I guess I shouldn't
have rubbed the tape so hard as I pushed in under the 45 a tad.
Regular masking would not need pins or rubbing so hard to keep in place.