I would be cautious about buying a kit for Stack and Whack especially if they are cut octagonal. I suggest you start with the first design - the pinwheel. Also a suggestion.
When you cut the individual sections, put them in a paper plate to keep them in order. Then all the other pieces for that block in the plate. This helps keep it all organized.
It is not difficult - more addictive than difficult. You just need for the pinwheel 8 repeats of fabric. Don't assume you can get repeats across the fabric but you MUST cut down due to slight differences in printing. You can use the other side of the fabric for other blocks or to enlarge the initial. You might also consider buying Bethahy's pattern for the pinwheel rather than start with the book. I found this easier to understand the concept without having to decide on what arrangement/pattern to make. The Pinwheel was her first and basic. All others play off that one.