Get the mantra from an authentic Guru. A renunciant, a mahatma, holy man, not a worldly person
If you don’t have a Guru, use the mantra ‘Ram’. Or mahamantra Hare Krishna Hare Krishna Krishna Krishna Hare Hare, Hare Ram Hare Ram Ram Ram Hare Hare
Use a mala/ rosary of 108 beads. Rudraksha for Kaali/ Durga/ Shiva mantras, tulasi for Krishna/ Ram/ Vishnu/ Narayana/ Lakshmi/ Hanuman mantras
Keep the mantra a secret, do not reveal it to others
Use a woolen blanket as an asana. Do not sit on bare ground. Don’t use the asana for any other purpose, fold it and secure it after chanting/ japa.
Face East or North while chanting.