How do you use tea tree oil to treat head lice? Do you put it on the scalp, add it to the shampoo?

Not recommended. Tea tree oil (melaleuca) is purported to be useful against fungi and bacteria. But there is no scientific evidence it is helpful for those or head lice. And if oxidized and applied on skin, or ingested, it can be quite dangerous to children.
Rid. There ardmore specific treatments for lic such as rid, 1-23 , ovode. Tea tree oil has an effect on bacteria and fungi. It could be used as an addition to shampoo after using products intended for lice.