It's all in stops (EV). Remember that a stop is a halving or doubling of the light. So, the manual power settings on a flash are a full stop scale (1, 1/2, 1/4, 1/8, etc.). Look at the aperture setting you want to use in the camera. Look at the reading the meter gives you. Whatever the difference is between those two settings? That's how much of an adjustment you need to make on the flash. For example, you want to be at f/4, but the meter reading is f/8. Then you want to bring the flash power down by two stops (e.g., going from 1/4 power to 1/16 power).

Neil van Nierkerk teaches a good trick, if you're not good at juggling f-numbers in your head. You can just use the camera itself to tell you how big an adjustment you need to make by adjusting the aperture from where you want it to be to what the meter's reading out, and counting the clicks. Then put the setting back where you want it on the camera, and adjust the flash's power output by the same number of "clicks" (just make sure the meter, camera, and flash are all set to use 1/3-stop adjustments).

See also: What does f-stop mean?