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.
Note: The 45° angle is the angle on the corners of a square. If you are making a quilt with odd shapes, such as a hexagon, you will need another angle to miter, but the procedure is the same.
When you are sewing the border strips on for a mitered corner, you must add extra length for the miter. I write this out each time, including the size for each side. This reminds me that there is a border on both sides that I must take into account. I don't want to say I have a short attention span, but when I just add in my head (quilt top + border), I invariably add the border from one side only.
For a mitered seam, you must add another 2" on EACH side for the miter. If your main quilt is 36" and the border is 2" wide, the math would look like this:
miter + border width + body of quilt + border width + miter + seam allowance
(the seam allowance is 1/2", which is 1/4" on each side and needs to be added only ONCE for the entire piece)
2" + 2" + 36" + 2" + 2" + 1/2" seam allowance = 44 1/2"
If you think of it in small steps, one section at a time, it is not difficult at all. I actually draw myself a little sketch, adding each border, and then I can visually SEE that there need to be TWO sets of numbers added, one for each side.
Attaching borders to form mitered corners is done differently from butted borders. You begin by sewing all four borders onto the quilt. Each border piece is marked so that the amount you have left for the border and miter is not sewn to the body of the quilt.
Remember to follow the rules for attaching borders. Mark the center of the border and the quilt and pin. Then pin the two ends (in this case, the mark you have made near the end), continue by pinning the center sections.
Strip extends past body of quilt
White mark on left measures the border width, seam allowance and amount for miter; arrow on left shows pin put in from left to right, well out of the seam allowance; right arrow shows seam allowance pressed open to reduce bulk
In other words, in the example above, you would mark the border strip 4 1/4" from each end. The seam allowance belongs to the main quilt BUT actually is not sewn. When you sew, you will have the border width, seam allowance and miter amount (4 1/4") hanging loose at the ends.
Stitch to within 1/4" of the edge. Take a look at the corner.
The two borders in the picture are in different colors so you can clearly see that they come together at a right angle. To make this angle, you have to work on the right side (rotating the quilt so the corner you need is always to the upper right).
Take the side border and bring it to the top, matching the top edges and having right sides together. This should put a fold exactly in the middle of the quilt corner at a 45° angle. Using your long ruler, place the 45° angle across the top of the border, so you can draw a line along the edge of the ruler from the corner of the borders to the intersection with the quilt corner as shown in the diagram.
Stitch from the quilt out to the edge, starting your stitch in the last stitch you used to attach the borders. Open and check that there is no gap in the stitching and the end pieces come out evenly. Press. Then (and only then!) trim away excess. Repeat this same procedure on all four corners.
Tip: If you have more than one strip in your border, sew them together and attach them as a unit. Carefully match the seams of the border strips as they are face to face and open it to check before sewing. To do this, insert the pin in the same direction as stitches would be sewn, then fold open the strips. If in any doubt, baste first.
By not trimming the excess fabric until you are satisfied, you are completely free to do this over. Once cut, it is almost impossible to make corrections.
The arrow line shows where you would stitch from the edge of the quilt top to the outer edge of the borders. The quilt top is folded right sides together to form a 45° angle.
Press with tails to one side. Trim.