How would I make a mitered garter stitch edging in the round?

[knitknitter]

I am making one of those ubiquitous shawls with sleeves. Now that I am almost finished with the body of the shawl, I would like to pick up and knit all of the stitches around the shawl - which is basically a knitted rectangle and make a mitered garter stitch edging. Can someone please tell me how I would do this? I am concerned about how I would make mitered corners and a neat-looking garter stitch edging. I basically trying to knit a seamless mitered edging in the round, but I am going around a rectangular knitted objected. Thanks to those who respond with advice about this.