The first thing you need to know is that the fabric you cut for your border needs to be longer by at least 10 inches than the desired finished measurements of your quilt.
Example: If you want your finished quilt to be 45x45 then you need at least 55 in long strips for the borders
Second thing is to remember not to cross over seam allowances. That is where the measuring and marking come in.
So let's get started
1. cut your 4 border strips the width you desire and at least 10 inches longer than the desired finished dimentions of your quilt.
3. Keeping border and quilt top right sides together fold at the corner (right side of quilt top showing) and mark a sewing line that is at a 45 degree angle from the seam line and from where the seam line ended (remember not to cross seam lines).
4. sew on marked line, open, press and trim excess frabric
And now you have a very nice clean mitered border.