What to say:
1. Before starting — Begin with "Bismillah" (In the name of Allah). This is the same invocation used at the start of ablution [Dua: Invocation au début des ablutions].
2. The intention (niyyah) — This is in the heart, not spoken aloud. You simply intend to perform ghusl to purify yourself from major impurity.
3. After completing ghusl — It is recommended to recite the same dua said after wudu:
*"Ashhadu an la ilaha illa-llah wahdahu la sharika lah, wa ashhadu anna Muhammadan `abduhu wa rasuluh."*
("I bear witness that none has the right to be worshipped except Allah, alone without partner, and I bear witness that Muhammad is His slave and Messenger.") [Dua: Invocation après avoir accompli les ablutions]
How the Prophet ﷺ performed ghusl:
The best guide is the description narrated by Aisha (may Allah be pleased with her): the Prophet ﷺ would first wash his hands, then perform wudu exactly as for prayer, then run his wet fingers through his hair until the roots were wet, then pour water over his head three times, and finally wash the rest of his body [Sahih Bukhari 1:63 (272, 273)].
Maymunah (may Allah be pleased with her) added that he would wash his private parts first, then strike his hand on the ground (or wall) to clean it, then perform wudu, then pour water over his whole body, and finally wash his feet in a separate spot [Sahih Bukhari 1:63 (276), Sahih Muslim 1:254 (317)].
Practical tip: Keep it simple — say "Bismillah," make your intention in your heart, follow the Prophet's ﷺ steps, and end with the shahada dua. You can find this and other invocations in the Invocations section of your Al Muslim Plus app to help you memorize them.