Start training gently as soon as the plants are tall enough to be tied with a bit of soft yarn, if that's the way you want to roll.
Pinch any time after the 5th node and before 2 weeks prior to flowering.

Just remove the growing tip, and 2 or more (depending on if you get the WHOLE tip or just MOST of it) new shoots will grow in its place. Leaving the very base of the growing tip gives extra shoots and is the FIM technique. It's done to make a plant bushy and shorter than it would have been un-pinched, and to provide MORE main shoots that you can then take as cuttings and turn into your new crop.