If one person decides to convert to Islam from another religion, can that person declare himself as Muslim, and be Muslim, or Is there specific rituals/ways when converting to Islam?
To become / convert a Muslim the person has to accept The Declaration of the Testimony also known as Shahada
For a Muslim, every action begins with your intention. You must put it in your heart that you want to strictly follow Islam and its rules.
It is not necessary to say in front of a crowd. You can quietly, to yourself, make the intention to embrace Islam as your faith.
Say the following words with clarity of intention, firm faith and belief:
Ash-hadu an la ilaha ill Allah.
I bear witness that there is no deity but Allah.
Wa ash-hadu ana Muhammad ar-rasullallah.
And I bear witness that Muhammad is the Messenger of Allah.
La ilah illa Allah, Muhammad rasoolu Allah. أشهد أن لا إله إلا الله و أسهد أن محمد رسول الله
I testify that there is no true god (deity) but God (Allah), and that Muhammad is a Messenger (Prophet) of God.
All a person has to do to convert is to say, with full belief he is saying the following,
أشهد أن لا إله إلا الله و أسهد أن محمد رسول الله
I witness that there is no god but Allah and Muhammad is His Messenger
This is preferred to be said in front of a group of Muslims so that they can be witnesses that you have taken the Shahadda and become Muslim, but you do not have to if you are not able to or something, you can do it at your home, after that you can declare yourself a Muslim.