This may be one of those projects that is mostly trial and error, so let me recommend using a lifeline. The concept is simple: basically you insert a piece of contrasting yarn through a row (or round) of stitches that you know is correct (and before you get to the complicated part). Then if you mess up and have to pull out your work, you won't be able to unravel it past this row (or round).
I think this video might tell you how to insert a lifeline for loom knitting: http://www.youtube.com/watch?v=vcoLV...BOE-gNBAVj-oGk
. I haven't watched it because I have to run, so check it out and see if it makes sense.