How to add borders that are flat.....
Lay your quilt on a large flat surface. Lay your border down the center of your quilt top. Cut the border to match the length of your quilt top. Repeat to get your second border strip.
Fold the quilt in half and place a pin in the center. Do the same for the border fabric. Now, pin the boder fabric to the side of the quilt at this center mark and one at each end. Distribute the fabric evenly. Sew the border in place. Press flat.
Repeat the above for the third and forth borders.
I know this sounds like a lot of extra work and by the time you get to this point you just want it done. You will be much happier with the results of your quilt if you follow these instructions. The last thing you want after you have spent so much time making your beautiful quilt top is to have to have tucks taken because there were wavy borders and I had no other choice. Your quilter will be happier too!