Place the mic as close to mid sternum as possible, generally about 6-8 inches from the source (or the chin). This placement should be below the shoulder line. Avoid placing the mic too high on the chest by the throat because frequencies can be blocked by the chin and cause the sound to be muffled or "muddy."  Keep in mind that if the mic is placed too far either side of center, there can volume changes as the speaker move their head from side to side.  See diagram below for guidance (1).  

NOTE: Hand holding a lavalier mic will produce inconsistent volume or audio drop outs, due to hand movement and voice fluctuations.

Do a sound check and adjust the volume of the sound if necessary for your voice.

Adjust the mic position if the quality of the sound is poor and repeat the previous step until you reach satisfactory audio