Once the core is prepared — I use rice hulls in a section of a new stocking or sock to form a ball, like a snow ball — the next step is to wrap it. First it is wrapped with yarn, then with thread. The size I want my finished temari to be is what dictates both the amount of rice hulls I use (anywhere from 3/4 cup to 1.5 cups, for the temari to date in this blog), and also the amount of yarn.
Using extremely soft yarn, I wrap the core gently and randomly until it is entirely covered. The wrapping must be random and loose to keep the shape round and free of lumps.
The next step is to wrap thread over the yarn. Regular sewing machine thread, in the color that will be the [back]ground of the temari. The thread must also be wrapped randomly without bunching or having lots of parallel strands, and only moderate tension should be used. If it is too tight, it will be impossible to sew through. The end result should result in a "springy," round-as-possible temari. The thread will compress the yarn, making the finished temari somewhat smaller in circumference than the yarn-wrapped stage.
While this is the most laborious (and sometimes tedious) step of temari making, it is also incredibly rewarding. Since one must concentrate to wrap randomly yet consistently, one can regard it as a zen-like task, a "meditational" task. The result — a mari base made from scratch and ready to embroider — is so worth it.
For an even more elaborate description of the process, visit the excellent site TemariKai.
(But not simple.)