I wanted to share with you how I make wavy borders. It's a great technique, and I love how it gives you a little mitred corner look. This technique is also used in the Modern Medallion quilt. My instructions below are for a smaller block, however you can adapt it to any size, and further instructions are provided in the Modern Medallion pattern for a larger quilt.
Fabric and cutting guide: 3½" strips of inside border fabric
 3½" strips of outside border fabric
Attach two inside border strips to the top and bottom of your block. Trim back to the block size. Repeat with the sides of the block to complete your inside border.
Cut the outside border strips back so they are just a little bit longer than you block size. Place the strip down on your rotary mat, with the block on top, both right side up.
Cut a gentle wavy curve with the rotary blade. Discard the outer top of the inside border.
Step four:Sew the inside border to the outside border, down the curved seam (you can pin if you like, however I usually don’t). When you are looking at your curve, it will looks wrong. It's a curve and it's supposed to! Don't worry, it will make sense once you have sewn it! Gently manipulate the curve to fit and press well. Spray starch if needed to get it lovely and flat.
When I sew my curves, I use the bottom fabric as a guide and hold the top fabric up and move it as I go. Doing it this way, means I don't need pins and have a lot more flexibility.
Step five:Repeat steps three and four on the remaining three sides.
Step six:Trim the lock back to the desired size and you're done!
I hope you enjoyed this tutorial. It's really not too difficult. Take your time, be gentle and back yourself.