The lav mic must be close enough to your mouth that it picks up sounds clearly but not so close that plosives and breathing noise becomes an issue.Position the mic between 6 and 8 inches away from your mouth as centrally as possible. Most lav mics are omnidirectional so you can get away with having them slightly off axis. For the same reason you can move your head a small amount when you talk. Even still, the more central the mic is and the less you move your head, the better the resulting audio.