The toughest thing was getting a reliable bit of time each day to sit down and do it. Routine, cajoling, and rewards were all involved. So was keeping it lighthearted; the kid has to be on board! Each lesson has straightforward exercises then a brief story, very short at first, longer later in the book. We'd do the exercises and one read of the story, then kid would read the story to my partner. We started in September, and I remember by Halloween the kid was reading candy wrappers. After finishing it, the next big thing was finding stories the kid genuinely liked to keep it going. Continuing to read together after the lessons ended helped: for a while, kids will keep running into lots of new exceptions to the usual rules, etc.
English spelling and pronunciation are a lot, and the book is also, implicitly, a catalog of the tricks English plays on kids and other learners. Part of the book uses a semi-phonetic alphabet where e.g. ee and sh/ch/th have distinct glyphs, but it all still looks enough like English that the jump to regular writing later in the book is doable for the kid. Even with that alphabet, the book has to teach common words like "is" and "was" as exceptions (with s sounding like z). Decades later one can forget little kids deal with all this and eventually handle it like second nature.
The book's originator thought that you could teach math with a broadly similar approach--breaking things down into very small steps and practicing them in isolation then in larger tasks--and doing that was part of his career, but I haven't found similar teach-your-kid book for arithmetic/basic math. If such a book did exist I'd've given it a try!