Welcome to the Quilting Board!
This helped me too. Thanks for such clear advice folks!
There is no fireside like your own fireside.
I have been mitering all day. I did four blocks with mitered sashing and on the 11th one, I had a problem. I think I got sloppy. I fold the sashing together, use the 45 degree line on the ruler and line that up along the top of the sashing, then draw the line with pencil. You do have to start and stop exactly 1/4" from the edge and then on the miter. I'm glad I am done, how about you? lo