I don't often add borders to my quilts, but I did add one to the Batman quilt I made for my daughter, to make it a little bigger. Plus, I was at a quilt retreat with a bunch of traditional quilters and they egged me on with it.
ANYWAY, I just saw a blog post about how you'd go about adding borders and it's exactly the way my Mum taught me to do it, so of course I like it.
Here's the link.
Have a look and give it a try. I find that this process does the squaring-up for me, so I don't have to worry about trimming a huge rectangle to square, with the difficulties that that presents.