You have multiple borders?
You sew the different fabrics together first and then apply them to the quilt center as one border.
You can put all four borders on and then do all the miters, or you can work your way around the quilt.
(I put them all on and then do the miters one at a time - just a personal preference. The end result is the same. ;-) )
Just make sure that you make them long enough so you have enough (and then some for wiggle room) to do the miter.
Here's a video by Sharon Schambers on mitering. She's working on a much smaller top than you have, but you'll get the idea. And you don't have to use her exact method, but it will give you a great guide on the process.
This is Part 1 (there are 4 parts total).
http://www.youtube.com/watch?v=zAAa66kJRr0
The thing you'll have to watch out for is that the different borders match up properly at the miters.
I haven't viewed them in a while, but I'm sure there is glue involved. :mrgreen: