This blog references a Missouri Star tutorial, so you might want to look there, too.
http://stitchnsing.blogspot.com/2012...den-plate.html
This one uses a pieced wedge; you would just ignore that for the look you are after:
http://lurlineg.blogspot.com/2009/12...-tutorial.html
There's a tutorial right in the tutes section! Not necessarily for a placemat, but for the Dresden Plate:
http://www.quiltingboard.com/tutoria...al-t80596.html