For this quilt I used a 3" wide green inner border, and a 5" wide outer purple border.
After sewing your blocks together into a top, iron your top nice and flat, and measure through the center of the quilt (as opposed to placing the ruler on an outside edge to measure - sometimes the outside edges stretch and cause you to add a little more than you need), the width and length of the top.
Your top at this point should measure 48.5" x 60.5". You will need to cut 2 border strips measuring 3.5" x 60.5". In my case, for both sides, I cut two strips of 40" wide fabric, from selvege to selvege, then pieced them together to make one long 3.5" x 80" strip. Then I cut it down to 60.5", and put leftover green in my scrap bin. Sew a border to the longest sides of your top.
Your top should now measure, with the two longest sides bordered, 54.5" x 60.5".
Next, you'll want to cut 2 borders 3.5" x 54.5". If you are cutting your borders from a fabric piece going selvege to selvege, you may again have to piece together a piece to 54.5".
Now, your top should measure, 54.5" x 66.5".
Using the same method as you did for the green border, you'll now want to add the 5" wide purple border. Cut 2 strips, piece if necessary, to 5.5" x 66.5". Sew one strip to each long side of your top.
To finish the top, cut 2 strips, piece if necessary, to 5.5" x 64.5". Sew one strip to each shorter side of your top.
The finished size of your top should be 64.5" x 76.5", before quilting or binding.