Purity: Method of Ghusl for a Corpse
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 553:
[The method of performing] the ghusl given to a corpse is the same as that of the ghusl for janābah; and the obligatory precaution is that one must not give a corpse immersive ghusl if it is possible to give it sequential ghusl. If sequential ghusl is given, the right side must be washed before the left side.
-Ayatullah Sistani, Practical Laws of Islam, Purification (Taharah)