I would like to stop during the library preparation procedure. When can I do that?

The only possible stopping point is after the Repli-G amplification (point 13 in the quick protocol). Program a thermocycler to run the REPLI-g amplification (30°C for 2 hours), then stop reaction (65° for 5 min) and put the cycler on hold at 4°C. The reaction can be put on hold overnight.