I saw it demo'd recently. Let's say you have all your squares down; you pinch the interfacing so that the fabric is right sides together (it's pre-perforated) and you sew a quarter inch from the edge of the interfacing. You do this in parallel rows for the whole sheet. Then, you go back and clip all the intersections so you can pinch the rows going in the other direction. You sew all those quarter inch seams.
I think in the demo she removed the interfacing at this point (which would mean you could hand sew) but I'm not positive about this (and it would be a lot of picking away the interfacing).
Hope that helps.