Much like rubbing your tummy and tapping your head, pouring latte art requires that you do two things at the same time. Pour the milk at a consistent and even rate AND shake the pitcher side to side with the even tempo of a metronome.
Use a wide mouth cup. Ideally I like a smaller size (6oz) but some might find a larger 12oz size to work better. The trick is with the wide mouth you will more easily see the design develop and if anything the wide mouth can assist in its development.
Here's something you might not want to do, but should: Practice with water first. It doesn’t have the same viscosity of milk but it can give you a chance to get a feel for pouring and then shaking at the same time. You will also need to be gradually but steadily raising the pitcher so that the milk continues to pour at a steady rate. Later in the pour there is less milk in the pitcher and to keep the milk flowing you will need to tilt the bottom of the pitcher up.
Getting back to the practical, you're ready to pour, and you need to position. Hold the cup on a slight angle, with the back of the cup being raised up and the edge of the cup closest to you sitting slightly lower. This fans the coffee out in the cup and helps in the development of the leaves for our Rosetta.
Pour starting in the center of the coffee, especially for small cups. Just start pouring straight into the middle of the coffee.
With the cup about halfway to 3/4 full give the pitcher a little side to side shake and you should start to see the leaves of the penumbra begin to form. The side to side motion needs to be more rhythmical, almost lazy, much like the casting of a fly line. Be patient and let the milk set the timing of the osciallations.
Continue the shake, continuing to pour in the center of the coffee. The leaves should move away from you on the surface of the espresso. After about 4-6 shakes you will need to begin moving the pitcher back towards you, continuing to shake side to side with a little bit of a tighter oscillation.
This movement is slower than what many people attempt initially. Don’t get nervous and try to rush things. It won’t work. Slow, steady, almost "natural" slow beat metronome movements are your goal.
As you near the edge of the cup having created lots of leaves or delineations in the surface of the espresso you want to then draw through those leaves with the pour of the milk. Do this slowly, and also elevate your pour just a bit to keep the center stem slim and complimentary to the leaves.
Do it too quickly and it will pull the leaves up tight making your Rosetta look like a Christmas tree that hasn’t had its branches come down yet.
Last bit of advice: Practice, practice, practice. Pro Baristi pour hundreds of drinks a day, and that's their practice time. You have the luxury of no lineups to deal with. Use it.
Image Sources: www.milindspandit.org and Coffee Geek and Information Source: Coffee Geek