Of course you have expertly pulled espresso and perfectly steamed milk ready, right? Good. Let's pour.
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
To begin, we have to lay a foundation of liquid milk underneath the crema. Start by pouring a thin stream of milk from about three inches above the top rim of your mug. The thin, liquidy milk will sink below the foamy coffee and create a supporting base for it in the bowl of the mug.
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
After you've pierced the crema, continue to pour slowly until the mug is about a third to half full. Then, steadily bring your pitcher down all the way, resting it against the cup as you continue to pour.
Increase the Flow
Once you make contact with the cup against the pitcher, increase the volume of milk you're pouring out. (The thickness should go from something like a mouse's tail to something like an opossum's tail.)
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.
When you bring the pitcher down low and increase your flow, you should see a dot, or halo, of white foam collect on the top of the coffee. This is your artistic belly-flop, and is also the genesis of the rosetta.
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.
Continue to gently rock the pitcher side to side using the "pumping" action from the last slide while increasing the volume of your pour.
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
Once you reach the far end of the cup and the top of your rosetta, start pouring a little less milk volume out of your pitcher. You'll need the last little bit of liquid for the final step, which will bring the whole shebang together at last.
To turn the waves into petals, lift the pitcher off the side of the cup and back up on your pour a little, while moving the milk stream forward, toward your thumb and the wider "leaves."
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.
The greatest thing about latte art might be the fact that once you learn how to do it, it comes as naturally as drinking the result. Just try not to pour a rosetta or heart once you get it down, and you'll find it virtually physically impossible. (Oh, well!)