Holy God, why does no one give a descriptive answer?
Look dude, to clone you need a growing tip. Not a leaf, not a 'branch', but a place of continuing new growth. Cloning is essentially a transplant of newly grown material. Branches keep sprouting more and more growing tips the longer they're in veg.
Right now, that plant has only one fully developed growing tip. The top. You can see two per internode for a total of 4 developing. The tiny light green leaves? Those are going to push upwards and sprout more and more the longer they go. Once those are grown out to having some small leaves of their own, they're ready to be taken.
Remember, growth is exponential. Two tips will grow four more, then four more, etc. Those four will grow eight. Mother plants stay in vegetative partially to prevent flowering (obviously), but primarily to increase the amount of potential clones.
This will seem very basic once your plants develop a little more if its a bit muddled at the moment. It doesn't matter when you choose to take the clones, the question you have to ask yourself is how many you want. After you know that, just allow the plants to get there baby.
Good luck,
-Grizz
