How do you actually run a cycle?
Write the sentence in the present, short enough to copy without resenting it. Keep one notebook and one page per day. Morning three, midday six, evening nine, in that order, and read each line rather than scribbling on autopilot. If the wording starts feeling false, note why in the margin and only change it between cycles. Add one small real action each week so the sentence stays connected to your life.
What is the 10-10-10 rule inside the 369 method?
There is no canon here, and practitioners use the name for two different things. The first reading is arithmetic — ten lines in the morning, ten midday, ten at night, a flattened version of three, six and nine for people who want the same effort in every session. The second reading is the popular one online and it has nothing to do with time of day.