Please check the following website of Sayed Sistani:

https://www.sistani.org/english/qa/01198/

Question: How is a Tartibi (sequential) Ghusl done?
Answer: In this method of Ghusl, a person should first make a Niyyat (intention) for Ghusl. Thereafter one should first wash one’s head and neck, and thereafter the remaining parts of one’s body. It is better that one washes the right part of the body first and then the left part. Furthermore, based on obligatory precaution, it is not sufficient to make the intention of Ghusl when moving the head, neck or body while they are already under the flow of water, rather, the part that one wants to perform Ghusl on – in the event that it is already under the flow of water – must be taken out from under the flow of water and then washed with the intention of Ghusl.