I love love love love mitered borders, regardless of the fabric.
I think they almost always look better than straight borders.
Like most techniques, it's not that difficult, you just have to take your time.
There are many different methods - here's a link to Sharon Schambers video
https://www.youtube.com/watch?v=zAAa66kJRr0 it's got multiple parts.
Watch it through, even though it's a bit over the top, to see what she's trying to accomplish, and what you need to look out for.
Of course, it's easy for me to say it's over the top as I haven't won a 334 National Best of Show awards. ;-)
But it's always helpful to see how a master work her craft.
Then you can find some other sites online to see other methods.
Good luck - your quilt will look so much more "finished" with the miters.
Oh, and the width of the border strip would still be cut 3.5"