Prentiss L, Thompson illustrates the Do's and Don'ts of lavalier placement and why its important.
Its show and Tell time education link; https://youtu.be/hONzIqjFeT0
How far should a lapel mic be from your mouth?
However, as a general rule, it's recommended that you position the mic on the chest, about a hand-span distance (20-30cm) from the mouth. This will give you a natural, uncoloured sound.