After a little trial and error, it looks like these are the key things to do:
- Start filling the cup slowly without disturbing the crema, leaving a nice dark background to pour into.
- When you start getting close and releasing foam, make it a point to have the first layer of white be well-defined and not a fuzzy cloud. For me, this means deliberate rocking/shaking at the right time to release the foam, and not doing it too early. Pinch off the pour like you're doing a tulip.
- For the next part, it probably doesn't really matter if you want to end up with a tulip or rosetta or heart, but the big key is to push that second layer into the first, through a combination of pouring slightly faster and moving the pour forward while leveling out the cup. That will insure that the second layer rests more or less inside of the first, with the first thinning out and becoming an outer ring.
- Finish off the design as a tulip or whatever you choose.