/th/umb is a consonant made by sticking your tongue out just a little between the top and bottom teeth so that the front top teeth touch the top surface of your tongue. That is, as if gently biting your tongue. While your tongue is in this position you gently blow air out of your mouth over your tongue. This is an unvoiced, quiet sound. Just air is heard.
theatre, nothing fourth thank anything fifth thunder everything sixth thorn author seventh thin arithmetic path thigh bathtub both thought healthy mouth theme wealthy beneath third Athens earth three birthday tooth
Listen to various native speakers reading these sentences. Some at natural speed and some read more slowly so that you can listen more easily.