If you have a headset microphone, this should be easy to do. Your microphone should be a bit to the side and below your mouth (so the microphone won't pick-up your breathing), and no more than a half inch (1-2 cm) away.
A standalone microphone makes recording a little more difficult. It is very important to keep your mouth at the same distance from the microphone for the entire duration of the recording of one file. The same applies to a handheld microphone, try to be consistent in the way you hold it when recording the prompts for one file. It does not have to be the same distance from one file to another, but must be the same for the duration of one file.
> how do I adjust my microphone for recording speech?