Always, always "groom" your milk right before you pour it: To "groom" means to swirl it around in the pitcher so that it stays shiny on top, rather than acquiring the matte look that indicates that the milk and foam have started separating.
Some baristas also like to swirl their espresso, which allows them to incorporate the foamy surface crema evenly throughout the liquid and makes the whole thing an even shade of brown.
High and Slow
Think of the milk at this stage as being like an Olympic diver, making her body as thin as possible to pierce the surface of the pool water without creating ripples.
Bring It Down Low
Increase the Flow
The combination of increased volume, increased velocity (the more you pour, the faster the milk will move in the crema to create the design), and the relative distance between the milk and coffee will turn that diver into something more like a belly-flopper, which is what you want here: You need the white microfoam to splay on top of the brown espresso to create the rosetta, though you want to have more control over where it goes than just flop it willy-nilly on the surface.
To begin forming what will become your leaves, you should start moving the pitcher from side to side at this point. Be sure to do this by using your hand only, not your whole arm: You're not simply "painting" on top of the latte. Instead, imagine yourself riding a bicycle with hand-brakes, and "pump" the handle of the pitcher with your fist as though you were trying to slow your bike down on a hill.
Don't lift the pitcher yet: It should remain in contact with the cup throughout the rest of the design creation.
At the same time, start to drag the pitcher backward: The increased volume of the milk coming out of your pitcher will push the design forward toward the front of the cup, and your backwards-moving motion will allow it to taper at the top to create the rosetta's signature design.
Top of the Design
This pierces the milk foam and sinks the center line below the coffee, making a stem for the rosetta—not unlike dragging a knife through chocolate and vanilla cake batter to create a marble effect.