Hellou my friend, first of all I don't recommend you to top an auto but you could start to train her with LST and be ready for when she will develop internodes
About topping, even if I don't recommend it for autos, the best way is to top when you have at least 6 internodes, you want your branches to be strong so the plant can handle an heavy yield, you can too earlier but the risk is to have a weak plant :/
If you want to see more about topping you can check my diary to make an idea, good luck :)