More about borders
Measuring for Mitred borders is a little different, you need to measure through the centre of your completed top in the same way, place a pin in the border at the edge of your top, then allow the depth of the border + 2". So if your border is 6 " wide (or Deep) allow 8 " past the edge of your top. this will allow you to join the borders on a 45deg angle to give you the lovely "frame " effect.
Mitred borders are a must if you want to show off a nice border fabric and if you have multiple borders try joining the strips together 1st and treat the border as 1 piece, just be sure you allow enough on both ends to turn it back at a 45 deg angle.