Here is a little tutorial on designing applique borders... basically all the things I've learned so far.
Step 1: Miter your borders. If you really hate mitered borders you can join them the usual way but I've found this makes lining things up easier. And, YES, I attach my borders before I applique them. I'm sure some people would disagree with this but since my borders are very symmetric I find this is way easier than trimming them and attaching them later.
Step 2: Find the natural division in your design. Since my layout is 3 blocks x 3 blocks I divided my border design into thirds.
Step 3: Don't finalize your design until you have it in fabric! The above picture has my first design which was only on paper. Once I started making the applique shapes and placing them things looked a little empty.
Ahhh! Much better.
Step 4: Once you feel like your design is working, complete one side to check it. Everything looks ok here.
Step 5: Work in units. There are 2 units in this design, a corner unit and a side unit. I'm stitching them one unit at a time to make things easier.