He suggests working on each level until you've mastered it. He says it better than me ...
Once you’ve worked your way through the entire set of level one problems, go
back through them again. This time, you should be able to go through them with
greater speed and accuracy. You’ll probably remember some of them, and be able
to solve them instantly. Some of them may require that you work through them
again as described above, but will go more quickly. Others will take a just as
long as before, and still others will stump you all over again. Just make sure
that you work through them again as thoroughly as you need to solve the ones you
can and understand the ones you can’t.
Q: "How many times should I go through the same set?" A: Until you can go
through the whole set (preferably in one sitting, or at the very least in as few
sessions as possible) and score at least 90%. Then you’ll be ready to move to
the next level of problems. Repeat the process until you’ve worked through all
ten levels. There’s no set schedule for this – take as much time as it requires,
even if it’s a year or two or more, as long as you work through them thoroughly
and spend at least a little time on them every day. As you can probably tell,
it’s quality rather than speed or quantity that I’m advocating. Once you’ve
finished the entire collection, you’ll be ready for the advanced plan.
That's it. But I'm going to try to be a little more hasty about it ... I don't want to just meander through these 1000 problems and finish a in few years. The one thing I hope to avoid is being caught up on the same problem over and over again. I plan on identifying the problems that give me grief and memorize them. Then as I go back through the level, I won't be stumped. My "practice" will be going back to the "problem-child" problems. My "test" will be to go through the whole set without re-do's.
I passed the 700th problem last night. About 300 to go.