Here is a description of how I use Washable School Glue to miter borders. It makes miters simple! I especially like to use a mitered border on a large quilt because you can assemble the border sections and then add them to the top in one step.
First prepare the border sections. Because this quilt is a rectangle, I marked the shorter sections to keep track of placement.
Be sure your border lengths are the measurement of the side of your quilt, plus 2x the completed width of the border, plus some extra so that they will overlap and extend sufficiently.
Because a mitered border is a “Y” seam, when sewing the borders to the top, the sewing must stop 1/4″ before the edge of the top so that all seam allowances remain free. You can see this seam using the brown thread at the edge of the quilt top in the photo.
On this corner, the two adjacent border seams did not quite meet. I go back and correct these. It makes the mitering process much easier when this is done well. Sew all the borders onto the top and look to see that all the seam meet neatly at the corners but don't extend into the seam allowance. Iron the seam allowance toward the border.
Next lay the corner out neatly on the ironing board, with the quilt facing right side up. Be sure the quilt is laying flat and the borders are at a right angle to each other. The big ironing board makes this much easier. Note how the outer edges of the border cross each other on the diagonal line on the ruler (which also crosses the corner where the top meets the border) and how the top and side edges of the quilt top line up with the ruler. The neater you are here, the more square the corner will be.
Mark both borders at the outer point where the two borders cross each other.
Take the border section that is laying on top and begin to flip it under and to the left, forming the mitered angle at the corner. The edges of the two border sections will line up with each other and the extending sections (that will be eventually trimmed off) are now laying right sides together. Neaten up any seams that need to meet each other.
Line up the marks you made on the outer edges of the border.
Iron to hold the crease. Lift up the crease you just ironed and place a SMALL about of washable glue on the underside. Don’t put it right on the edge but rather, in from the edge as shown. You only need a little. Now carefully lay the border back in place, and iron to set the glue.
Now we get ready to sew!! The crease you just made and glued in place is the seam line. Lift that section so you expose the crease you just ironed. The glue will hold the border sections in place while you sew. If the ironed crease is hard to see, place a few marks along the ironed path.
Be sure that you don’t sew any seam allowances from the top into this seam. Remember that when you are sewing “Y” seams, you can only be sewing 2 pieces of fabric together, never any more!! Sew!! And do lock this seam where it meets the quilt top so it won’t open up.
After sewing the seam, pull the seam allowances apart (freeing up the glue) because you will want to be ironing the seam allowances open.
Then trim the seam allowances and iron open. Keep the seam allowances of the quilt top ironed toward the border. Be sure you haven’t caught any seam allowances in any of the border or miter seams. If you have caught any seam allowances, they may keep the miter from laying neatly.