Basically, I learned how to do it from the Madsen's Chainsaw website, and from other members on here, like Brian who posted a video of his sharpening of square file.
I also found this to be very helpful in understanding the angles and what needed to be done:
http://www.madsens1.com/bnc_cb_angles.htm
IMO, the two most important things with square are keeping the corner lined up, and the side plate (they cut the grain). Also, do not reduce the angle of the top plate too much, or the chain will not feed well (the top plate functions mostly as a chisel anyway, so the angle does not have to be that steep).