You're growing Autos so time is a big factor in topping.
I top and train as follows,
14 days after it sprouts/breaks ground, top the plant.
Once the new branches have grown a little, start LST. Should be able to start LST within 7 days of topping.
If you go to my Auto Tent #1 Diary, go to Week 2 Veg and you will see split pictures showing before and after topping.
Those pictures will give you a good idea on where to cut your plant.
Make sure to clean scissors with isopropyl alcohol before making the cut.
If you top/cut more then 1 plant, cean the scissors again before cutting another plant
Less chance of spreading any kind of bacteria, viruses,