DON'T PRE-WASH! Then just follow the instructions in the book. When you have sewn together three triangles to make your half hex, line it up next to a ruler or the edge of your mat. If it is straight, no cupping. If it isn't straight, re-sew. No problem.
Don't pick a fabric where the elements are too small. A medium print with movement in the background is best. Also go for the 24" repeat. You will have more variety in your quilt.