Topping

When should I start topping and how do I go about it?

I do it pretty early. 4-8 inches works for me. That way they get bushy before they get tall. I usually FIM my plants instead of topping them. Here is a video on how to top or FIM so you can decide for yourself.

http://www.youtube.com/watch?v=u0gr4Bez3Xg

I would personally wait until the 6th or 7th node is there which will help ensure you have a good root system for what you are about to ask the plant to do. But before you go topping or fimming a plant ask yourself what your goals are for the plant. Are you planning on super cropping?, just want 2 to 4 main colas, how long are you going to veg? You need a plan then explore the ways to acheive your desired results.
Here is a link for you to go through, it is very long with lots of opinions and many pictures.
Good Luck and Welcome to RIU enjoy your stay

https://www.rollitup.org/advanced-marijuana-cultivation/151706-uncle-bens-topping-technique-get.html