Please approach your nearest branch of Sri Ramakrishna Math and tell them about your intention of taking Mantra. The monk/ manager in the Math would normally ask you about your background knowledge about Sri Ramakrishna Paramahamsa, Sarada Devi and Swami Vivekananda. You are expected to have some reasonable exposure by reading their books and have a conviction about Sri Ramakrishna as a divine Avatar.
Mantra diksha is given only by one or two designated senior most monks of RK Math (President / Vice President of Belur Math) and as and when such a monk visits the branch (may be a once or twice in a year), you can get intimated. They will also display a board in the Math premises about the visit and giving of Mantra diksha. You have to fill up a form asking your biodata and about your interest on Ramakrishna. You can thus get registered to be a seeker of Diksha. They will intimate to you when and how to come and the things you have to bring in the morning. (Flower garland, fruits to offer, a pair of dhoti & angavastra to be given, Guru dakshina (as per your wish), any donation willingly given etc).
Please note: Ramakrishna Math will initiate you only in Sri Ramakrishna mantra. If you want to be initiated in any other mantra (like Shiva mantra or Narayana mantra) they won’t.
You are expected to come with empty stomach early in the morning.
The ceremony will be a common session for all the seekers of diksha for the day. There will be a bhajan session and then a formal puja to be performed by you etc. The Diksha guru would give a small introductory lecture; ask for any clarifications from the gathering. He will give the mantra commonly to all in a closed hall/ room. (not individually). Then each individual will be invited to come to the Guru and he will be asked to chant the mantra to the Guru so as to ensure that it has been grasped properly and pronounced properly.
As a regular practice, you will be advised to chant the mantras at least 108 times per session twice a day. The more you do, the better. An instruction slip will be given to you giving further guidelines on Mantra chanting, and advice of doing manasa puja, meditation etc.
Then you will individually offer a pada puja to the guru after garlanding him and making the offerings brought by you.
The session will end sometime in the noon. Immediately, you will be offered a glass of juice (Panakam) first and then a sumptuous prasadam to eat.