بسم الله الرحمن الرحيم
(Fatwa: 814/814=M/1429)
In the beginning, the Prophet (صلی اللہ علیہ وسلم) used to preach secretly. He started preaching the message of Islam to his near and dear ones. He went door to door to convince them about oneness of Allah, prophets and Akhirah. He did not hesitate to convey the message to heads of tribes. He used to be polite and wise in preaching. Generally, he approached people individually and as per the need he used to give collective Dawah as well. Once, he climbed upon the Safa Mount, called all tribes and invited them to Oneness of Allah. The Prophet (صلی اللہ علیہ وسلم) discharged the duty of Dawah through letters as well. For details, you should study books.
Allah knows Best!
Darul Ifta,
Darul Uloom Deoband