I would carefully figure out what the length of the outside border would be and add a few more inches. Make sure that each border is measured accurately and is stitched to that determined length. This is the place where MOST wonky borders occur.
I always had one corner off a tad so I would measure, baste, press and see if it was accurate. I used my 15" ruler and a couple of 24" ones to make sure everything lined up correctly. Once I was assured that every piece was correct, I would then do a normal stitch and then carefully trim the access. I didn't cut as close as a 1/4 seam and I press that seam open. I know that there was a F&P show that Porter did a quilt with like 5 or 6 strips sewn together for a border using the reproduction solids. Sorry, I can remember the pattern!!
PS. If you have sewn all your strips together, you treat it like one piece of fabric. You will need to be careful in making sure all your fabric seams line up correctly with the other seam on border you are attaching it too.