It is worked from the top-center out. So you start with a few stiches and increase ever-other row, once on each edge and twice in the middle. You cast off along the two diagnal edges. Guage does not realy matter here and if you are worried you do not have enough yarn it is easy to cast off earlier.
happy knitting

Also I am trying to get a pdf version up so you can download it, but I have to wait for tech suport.