Answered by Sayyidi al-Habib Umar bin Hafiz (may Allah protect him and benefit us by him).
What etiquettes should be observed when someone embraces Islam?
It is good that it be made public.
As for the wording, the person embracing Islam should be told to say:
أَشْهَدُ أَنْ لا إِلهَ إِلَّا اللهُ و أَشْهَدُ أَنَّ مُحَمَّداً رَسُولُ الله
Ashhadu alla ilaha ill’Allah wa ashhadu anna Muhammadan rasulullah
The meaning should be explained to him or her:
“I bear witness that there is no deity save Allah and I bear witness that Muhammad is the Messenger of Allah.”
If possible they should be told to say the following and the meaning should be explained:
آمَنْتُ بِاللهِ و مَلائِكَتِهِ و كُتُبِهِ و رُسُلِهِ و اليَوْمِ الآخِرِ و بِالْقَدَرِ خَيْرِهِ و شَرِّهِ مِنْ اللهِ تَعَالى
Āmantu billāh wa malāi`katihi wa kutubihi wa rusulihi wa’l-yawmi’l-ākhir wa b’il-qadari khayrihi and sharrihi min Allāhi ta`ālā
“I believe in Allah, His Angels, His Books, His Messengers, the Last Day and in pre-ordainment, whether it be good or bad from Allah Most High.”
رَضِيتُ بِاللهِ رَبّاً، وبِالإِسْلامِ دِيناً، و بمُحَمَّدٍ نَبِيَّاً ورَسُولاً
Raḍītu billāh rabban wa bil-Islāmi dīnan wa bi Muḥammadin nabiyan rasūlan
“I am content with Allah as a Lord, with Islam as a religion, and with Muhammad as a Prophet and Messenger.”