I bought two balls of James C. Brett Marble Chunky (in the Adobe colourway) and started by casting on 3 stitches, then increased one stitch per row until I'd run out of yarn from the first ball. Then I just had to attach the second ball and decrease until I got to the opposite corner. You don't have to worry about how wide or long the blanket is going to be to get the optimal usage out of your yarn because the thing will always turn out square.
I modified the first few rows by doing a knit-front-back-loop until I had 6 stitches, and then started the increases with the yarn overs as written. For the last few rows, I only did the yarn overs until 6 stitches remained and then I just decreased by doing one knit two together per row until I had 3 stitches left and bound off. I found it made for a much nicer square edge.
I was really digging the ribbon some people had threaded through the eyelets created by the yarn overs, and so decided to give that a try. After unsuccessfully knitting a really long icord and running out of yarn before it was long enough, I caved and bought a pretty green ribbon to use instead. In retrospect, it probably wasn't the greatest solution, and if I were to do it again, I would go for the icord. As it turns out, satin ribbon isn't very stretchy, and garter stitch fabric is. This means that whenever I'd move the blanket around the ribbon would slide around in the eyelets and the blanket would stretch in weird ways.
Besides the ribbon, however, I'm pretty happy with this blanket. The yarn is so soft and squishy, which is only accentuated by the garter stitch, and the colours are beautiful. I would probably make this pattern again.