“Ghusl” is a full bath (or shower), completely washing the entire body with the intention of entering into Islam. This is symbolic of washing away the previous mistakes and sins and preparing to be forgiven by Allah for everything that has occurred up to this point.
This is not mandatory, but is very good step and can also be done after the Shahadah as soon as possible.

It is preferable to have someone present (like Imam or ‘Alim) who knows about giving the Shahadah, but if not anyone can be witness if they are a mature Muslim and understand what is being said.

The person must understand what is being said and what it means to be a Muslim practicing true Islam and the peaceful life excepted of them.

First, to say the meaning of the Shahadah in their language or a language they can understand.

Important: If they don’t really understand the meaning this is serious.

Second, repeat the Shahadah in Arabic, out loud and in front of witnesses.

La ilaha illa Allah; Muhammadur-Rasul Allah. ‘There is no god but Allah; Muhammad is the Messenger of Allah.’
Pronunciation is very important. Be sure the Shahadah is said properly.

Simple way to think of it: “Worship the Creator – Not His Creation”

Declaration : “La ilaha illa Allah; Muhammadur-Rasul Allah”

This declaration contains two parts. The first part refers to God Almighty, the Creator of everything, the Lord of the Worlds; the second part refers to the Messenger, Muhammad (Peace and blessings be upon him) a prophet and a human being, who received the revelation through the Archangel Gabriel, and taught it to mankind.

By sincerely uttering the Shahadah the Muslim acknowledges Allah as the sole Creator of all, and the Supreme Authority over everything and everyone in the universe. Consequently the Muslim closes his/her heart and mind to loyalty, devotion and obedience to, trust in, reliance on, and worship of anything or anyone other than Allah. This rejection is not confined merely to pagan gods and goddesses of wood and stone and created by human hands and imaginations; this rejection must extend to all other conceptions, superstitions, ideologies, ways of life, and authority figures that claim supreme devotion, loyalty, trust, love, obedience or worship. This entails, for example, the rejection of belief in such common things as astrology, palm reading, good luck charms, fortune-telling and psychic readings, in addition to praying at shrines or graves of “saints”, asking the dead souls to intercede for them with Allah. There are no intercessors in Islam, nor any class of clergy as such; a Muslim prays directly and exclusively to Allah.

Belief in the prophethood of Muhammad (Peace and blessings be upon him) entails belief in the guidance brought by him and contained in his Sunnah (traditions of his sayings and actions), and demands of the Muslim the intention to follow his guidance faithfully. Muhammad (Peace and blessings be upon him) was also a human being, a man with feelings and emotions, who ate, drank and slept, and was born and died, like other men. He had a pure and upright nature, extraordinary righteousness, and an unwavering faith in Allah and commitment to Islam, but he was not divine. Muslims do not pray to him, not even as an intercessor, and Muslims abhor the terms “Mohammedan” and “Mohammedanism”