Before I start, I'd like to remind you that as soon as you injure your plant (by toping, for example), your training falls into the high-stress category, even if you do trellising afterwards.
In your case, you need to wait a little longer, until the plant has grown enough to be able to do a toping. The first 4 nodes (3 if you're mainlining) should have started to develop lateral branches and you can grow a bit more if you want to take a cutting at the top.
Prune the first node or 2 from the bottom, leaves and branches. The next 2 nodes are generally kept, but you can keep more if you like or just one if you intend to mainline. Cut the main stem cleanly, right between the two nodes. And even if the plant can get by without it, I recommend using tree-healing putty to cover the wound.
And remember that every time you top the plant, it will stop growing to deal with the stress, which will slow down your growth period by a week on average each time.
+2 strains
+1 strain
2 strains
