This is gonna be a real quick rundown on topping. I will usually top mine after the 4th or 5th node. I make my cut about an eighth of an inch above the node. Cutting just the main stem. The new growth coming from the node will become your two tops. NOTE: Always make sure your tools are clean. I use alcohol rubbing pads to clean them before and after I use them.
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That's pretty much all there is to it. Now if you want to clone your tops i would suggest to cut it another node down the plant. This will give you more stem to work with.