I think I had the same issue. An L-seam looks to be any (horizontal line segment, vertical line segment) pair having a line segment endpoint in common, at least one of which could not be cut by any sequence of guillotine (end-to-end) cuts. So the (minimum possible) number of L-seams is well defined for a given problem instance, but not (necessarily) their specific locations.
Yes, I think the idea is that you start with the individual squares, and you can sew two pieces together at a time. If you come to the point where you need to join two pieces along more than one edge, you have an L-seam.
