To cut your borders, add the length of the border plus your width. I even add extra for overlap. For example, I want an 8" border on a quilt that is 34" X 34", I would cut (4) pieces of fabric 8½" (includes SA) x 54" (34"+8+8+4" [the 4" is for overlap])

Match the centers of the border with the center of the quilt and stitch in ¼" seam. Sew all four. It should look like this:

When you have the borders pressed there are sections that will overlap. Trim the underneath part evenly to the border as illustrated below.

Place the trimmed part over the other side where it overlaps and trim. See below.

One corner should look square. Repeat with the other 3 sides.

If you place the corners of the borders right side together, the edges should align. It might look slightly distorted at this time but draw a line from where you stitched the ¼" seam to the outside corner as shown by the red line on illustration. Stitch along this line and trim ¼" away as shown by the blue line. Open seam where you just stitched and press.