I did it a bit with dc1. I found it helpful to give me pointers to a routine. When I hadn't a clue, it was a useful place to start.
One or two of the things she suggested were life savers.
BUT I have 3 dc, (all now 10+) and looking back at them and the method, I think dc1 naturally was a child who craved routine, and he also, right from birth fed like clockwork, every 3 hours on the dot.
It would not have worked with dc2 at all, and I never felt the need to try it, as she and I fell into our own routine, and her feeding was totally erratic.
Dc3 just gets to fit in around the rest of the family!
Things that helped me:
getting up at 7am to start the day, so you 'reset' the body clock every morning. This really worked because dc1 was very routine with his feeding, and a decent sleeper.
when dc1 at 5 weeks suddenly stopped falling asleep during the day, I read the bit where it says, 'a baby who has been awake for 2 hours, needs a sleep' and I tried it, and it worked. The longer I left it, the more hyper he became until he couldn't sleep and we had a screaming baby at 4 pm who was over tired.
This remained true all through his childhood - too late to bed = hyper and can't sleep!
But dc2 was different, if she was tired she didn't become hyper, she just went to sleep, so none of that was necessary for her.
I also found her feeding/ weaning schedule handy, later on.
What did NOT work for me was that she suggests top up feeds, can't remember the details, but my baby did not do top up feeds ever, if her wasn't 100% hungry, he was not going to eat.
As any book, take what works, and remember it was designed for ff babies, and GF is not a mother.