Does the Shahadah process include a full bath and circumcision? Should it be performed in a mosque with an Imam and witnesses present? Is it similar to adult baptism?
There are different approaches to this question. Some of the scholars who are more of a traditional bent, tend to prescribe that the person should take a bath either before or just after pronouncing the testimony of faith, which is what Shahadah means, and some will insist on circumcision.
Shahadah is the testimony of faith. There are two things that one testifies to declare oneself as being part of the Muslim community. That's the belief that there is only one God that we refer to as Allah, using the Arabic term, but one can say God with a capital G. Secondly, Prophet Muhammad (PBUH) is the messenger of God. He brought the message of God to the world, and Muslims are trying to follow that message. Muslims are going to follow Prophet Muhammad (PBUH) as a model for how we ought to be as human beings. Those are the two declarations of faith.
These declarations would be our pronouncement that we're entering the Muslim community. One might've already been a Muslim at heart prior to this declaration. Meaning, the person can be a Muslim just between that person and God, but we don't want that to be for too long. Since that could be like an intermediary stage. One might be learning about Islam over the years, there comes a point where the person wants to join the Muslim community. The person who’s a Muslim at heart.
Theoretically, if this person passes away, then that person dies as a Muslim. Since only God knows that this person is a Muslim. God is the ultimate judge. For example, if a person wants to be part of the Muslim community, then one needs to be known as a Muslim to the community to make oneself known. One comes and makes this declaration public by bringing two witnesses. Maybe the leader of faith in the Muslim institution, plus one other witness at the time would solidify this.
Yet, it is a way of making a solemn declaration. For example, one is at home thinking, "I'm a Muslim now," by conviction. However, one might change one's mind the next day. Yet, when one comes into the Muslim institution, makes that solemn declaration, and in the presence of witnesses. This is a commitment and it's being pronounced its commitment in words. That's what one expects that will solidify this person as a Muslim and a new member of the community.
It becomes the responsibility of the Muslim community to look after this person as a new member of our community, especially if a person is embracing Islam from an environment in which Islam was not very welcome. That person may face some resistance from one's family, friends, etc. Maybe some deprivation of usual financial benefits. The Muslim community should look after that person. It's a two-way street.
The bath is prescribed for certain ritual Islamic acts for prayer. However, it's not so clear that this is prescribed for the circumstance of becoming a Muslim. By the time of the next prayer, one wants to ensure that one is in a total state of cleanliness to enter the prayer. Yet, it's not so much for the pronouncement of the declaration of faith itself.
As for circumcision, it is a known Sunnah or regular practice for men in the faith. Yet, if a new person has come into the faith, circumcision isn’t the first obligation for the revert. The revert needs to be given time to become accustomed to the faith. Once that individual is ready for circumcision, it solidifies his entrance into the faith and his commitment to living according to the Prophet’s way.
Generally, circumcision is known as one of the practices of the great Prophets. Specifically, from the Bible and the book of Genesis that Prophet Abraham (AS) was circumcised. He circumcised his son and the people around him.
According to Luke, Jesus (AS) have been circumcised on the 8th day. For Muslims, this is an important practice, but it will be classified as a Sunnah, which means that it is a general practice. This is above the level of just recommendation, but it is below the level of obligatory. It's an easier matter to circumcise our newborn boys than to impose on a new member of the community, especially when they're advanced in years, that they should now become circumcised.
In fact, there is a website, Islam Web, that gives fatwas on several issues. They have a fatwa, for the new Muslim, it is not to be an imposition and they cite the "Tafsir of Al-Qurtubi", a famous book of commentary on the Quran. It cites Al-Hasan, the grandson of Prophet Muhammad (PBUH) as not the requirement for the new Muslim. Since it is a non-community practice, maybe a new Muslim may decide to get circumcised. That would be a good decision because it will be one of the practices that are known among the practices of the prophets. However, this point isn't needed to be emphasized to discourage somebody from becoming a Muslim.
In adult baptism, one is dipped in the water. The Muslim, in preparation for prayer, would wash one complete body of major impurities. Then, one will wash the extremities just in preparation for the prayer, so this will be an ongoing process. It happens daily. It's not a once and for all baptism.