The first point is that you will need to make the quilt borders longer than just the width of the quilt or panel. The panel shown is 10.1/2" wide and the border is 2.1/2" so I have cut the border strip 14.1/2" long. That's 10" for the quilt block and 2" extra each side. Find the middle of both the border and quilt block (by folding in half) and pin the quilt border in place.
With right sides together and using a 1/4" seam, sew the border to the top of the quilt block. Begin sewing 1/4" from the edge of the quilt block (on the left) and finish sewing 1/4" from the edge of the quilt block (on the right).
Repeat the procedure with a strip of border down the next edge of the quilt block (or quilt), again beginning and ending the stitching 1/4" from the edge.
Fold the quilt or quilt block along the diagonal with right sides together. Match up the 2 border strips so that they are in line with each other.
Line your ruler up along the fold line of the quilt, through the last stitch of the seam and across the border strips. Mark a line on the border strip following the line of the ruler.
Stitch along the line that you have marked, starting stitching on the last stitch of the seam line. Trim the excess triangles off about 1/4" from the stitching.