I extracted the answer from a translation I had made for a spanish speaking group. In other words, this is a rough version of what the manual says.
The manual says to have chapter 16 handy in Monroe's book, as a reference for details.
"Lay down, preferably when you're tired or sleepy. When you're about to go to sleep, focus your mental attention on something, whatever, with eyes closed. Keep your attention on the one thing without going to sleep.
When you hear "remember your purpose", you remember you're not going to let yourself sleep by thinking on this one thing." So yes, you pick whatever you're going to think about. And you can change it if you want provided it's not regular mental chatter, because that will lull you to sleep.