Hi! I also have that book and while I haven't made a quilt with them yet, I have made many many potholders. Anyway, as I understand it, you sew three triangles together for each side, but do not sew the center seam. Pin the two halves together so you don't lose the other, and put them on your design board. When you have the blocks arranged as you'd like, sew them into strips. That means you're sewing one half of a hexagon to the half that's directly above it. The seam will be diagonal to the row. Once you've sewn all the half hexagons together into strips, start sewing the strips together until you have one large quilt top.
Doing it this way, while you have to be careful to line up the centers of the hexagon halves when sewing the strips together, you never have to sew a y-seam and it's easy to keep the quilt square.
Hope that helped!