Begin incrementally adding the warm broth one ladle at a time, stirring frequently. Wait to add another ladle until the liquid has been almost completely absorbed by the rice. This gradual addition of liquid is key to getting the rice to release its starch and create its own delicious sauce, so don't rush this step. If the broth is evaporating very quickly, or the rice is boiling hard, turn the heat down so you just see a gentle simmer and occasional plop. Expect this stage to take about 18 minutes, and to add most of the broth.