Here are the basic directions I use for mitered squares. I put a stitch marker in the middle of an even number of stitches, then decrease before and after the marker by knit 2tog. So my cast on number of stitches must be an even number, and I decrease by 2 stitches, on every other row. I knit straight across with Color A, row 2 of color A when I get to 2 stitches before the marker I k2tog, pass the marker then k2tog and knit across the row. 2rows of each color, so then I switch to color B and knit across. Row 2 of color B will the decrease row.
It's pretty much the same as Mason Dixon's - they are just decreasing a little differently when they get to the middle of every other row. Because thier decrease method uses a pass slipped stitch over, they can't put a marker there to mark the center.
After 4-6 rows it is pretty obvious when you get to the center, I just like having the marker there so I don't have to pay so much attention. I don't have to count rows, either, to keep track of when to decrease as it is obvious if I'm on the 1st or 2nd row of the 2 colors.
the mitered squares in this skirt pattern are the same as the first link in this post - using a even number of cast on stitches and decrease by k2tog. http://www.magknits.com/Oct06/patterns/mitred.htm Sometimes looking at the same basic pattern written slightly differently can make it easier to understand.