Topping is done when you cut off the newest shoot of a node or top of your main stem. To start, I'd just top the shoots on the main stem and you will see two nodes will grow beside it in its place. I attached an image with a crude drawing so you get the gist of how it works.
IMO there is no reason to top your plants. If they are too tall then you can squeeze the stem for 20-30 seconds and it will bend over. Usually I will have multiple shoots come out of the bent branch. I like to bend them out like a star gives the canter ;ots of light. Ibelieve thay call it traing the plant. WM
I top my plants because I want to keep them small in height. Once I topped them they stopped upwards growth and focused growth on secondary growth. They really bush out and that is what I'm looking for. I do not have more then 3 ft. of grow height so I keep em small and bushy.