✅ Answer: It is necessary to observe the same manner and sequence that has been mentioned in the hadiths. Pay attention to the following narrations:

عَنْ مُحَمَّدِ بْنِ مُسْلِمٍ عَنْ أَحَدِهِمَا قَالَ سَأَلْتُهُ عَنْ غُسْلِ الْجَنَابَةِ فَقَالَ تَبْدَأُ بِکَفَّیْکَ فَتَغْسِلُهُمَا ثُمَّ تَغْسِلُ فَرْجَکَ ثُمَّ تَصُبُّ عَلَی رَأْسِکَ ثَلَاثاً ثُمَّ تَصُبُّ عَلَی سَائِرِ جَسَدِکَ مَرَّتَیْنِ فَمَا جَرَی عَلَیْهِ الْمَاءُ فَقَدْ طَهُرَ

Translation: Muhammad ibn Muslim said: I asked Imam Sadiq or Imam Baqir, peace be upon them, about ghusl al-janabah; the Imam said: Start with your two hands and wash them, then you wash your private parts and clean them, then you pour water over your head three times, then pour water over the rest of your body two times, so whatever water flows over it has become pure and clean (it has been washed).

✅ The presence of the word "thumma" (then) in the Arabic language is a sign of sequence, and it must be observed in all cases where it is used in the narrations of ghusl.

عَنْ أَبِی بَصِیرٍ قَالَ سَأَلْتُ أَبَا عَبْدِ اللَّهِ عَنْ غُسْلِ الْجَنَابَةِ فَقَالَ تَصُبُّ عَلَی یَدَیْکَ الْمَاءَ فَتَغْسِلُ کَفَّیْکَ ثُمَّ تُدْخِلُ یَدَکَ فَتَغْسِلُ فَرْجَکَ ثُمَّ تَتَمَضْمَضُ وَ تَسْتَنْشِقُ وَ تَصُبُّ الْمَاءَ عَلَی رَأْسِکَ ثَلَاثَ مَرَّاتٍ وَ تَغْسِلُ وَجْهَکَ وَ تُفِیضُ عَلَی جَسَدِکَ الْمَاءَ

Translation: Abi Basir said: I asked Imam Sadiq, peace be upon him, about ghusl al-janabah; the Imam said: You pour water on your two hands and wash your two hands, then you put your hand into the water (container) and wash your private parts, then you rinse your mouth and nose, and you pour water over your head three times and you wash your face (as well) and (then) you let the water flow over your body.

✅ In the two above narrations and a few other narrations, it is explicitly clear that the ghusl of the head precedes the ghusl of the body after the initial rituals of ghusl. After pouring three handfuls of water on the head and washing it, we wash the rest of the body in the following manner:

عَنْ حَرِیزٍ عَنْ زُرَارَةَ قَالَ قُلْتُ کَیْفَ یَغْتَسِلُ الْجُنُبُ فَقَالَ إِنْ لَمْ یَکُنْ أَصَابَ کَفَّهُ شَیْ ءٌ غَمَسَهَا فِی الْمَاءِ ثُمَّ بَدَأَ بِفَرْجِهِ فَأَنْقَاهُ بِثَلَاثِ غُرَفٍ ثُمَّ صَبَّ عَلَی رَأْسِهِ ثَلَاثَ أَکُفٍّ ثُمَّ صَبَّ عَلَی مَنْکِبِهِ الْأَیْمَنِ مَرَّتَیْنِ وَ عَلَی مَنْکِبِهِ الْأَیْسَرِ مَرَّتَیْنِ فَمَا جَرَی عَلَیْهِ الْمَاءُ فَقَدْ أَجْزَأَهُ

Translation: Zurarah said: I said (to the Imam): How does a person who is in a state of major ritual impurity perform ghusl? The Imam said: If no impurity has reached his hand, he submerges it in the water and begins with his private parts and cleans them with three handfuls of water, then he pours three handfuls of water on his head, then he pours water on his right shoulder two times and on his left shoulder two times as well, so whatever part of the body the water flows over, that is sufficient for him (he has been washed).

✅ In the above narration, after washing the head, the Imam's instruction is to pour water on the right shoulder twice and on the left shoulder twice, and any part of the body that the water reaches with these four handfuls is sufficient for ghusl according to the above narration, and any part that is not washed by this pouring of water, we reach it with our hands according to the second narration we mentioned and some other narrations, and the ghusl is completed.

One can also wash the body according to the following narration:

عَنْ أَبِی عَبْدِ اللَّهِ قَالَ إِذَا أَصَابَ الرَّجُلَ جَنَابَةٌ فَأَرَادَ الْغُسْلَ فَلْیُفْرِغْ عَلَی کَفَّیْهِ وَ لْیَغْسِلْهُمَا دُونَ الْمِرْفَقِ ثُمَّ یُدْخِلُ یَدَهُ فِی إِنَائِهِ ثُمَّ یَغْسِلُ فَرْجَهُ ثُمَّ لْیَصُبَّ عَلَی رَأْسِهِ ثَلَاثَ مَرَّاتٍ مِلْ ءَ کَفَّیْهِ ثُمَّ یَضْرِبُ بِکَفٍّ مِنْ مَاءٍ عَلَی صَدْرِهِ وَ کَفٍّ بَیْنَ کَتِفَیْهِ ثُمَّ یُفِیضُ الْمَاءَ عَلَی جَسَدِهِ کُلِّهِ

Translation: Imam Sadiq, peace be upon him, said: When a person becomes in a state of major ritual impurity and wants to perform ghusl, he should pour water on his two hands and wash them up to the elbows, then he puts his hand into his container and washes his private parts, then he pours water on his head three times with two full handfuls (and washes the whole head), then he pours one handful of water on his chest and one handful of water between his two shoulder blades and then he lets the water reach all of his body.

✅ After pouring water on the body, we use our hands to make the ghusl water reach the places it has not reached.

✅ Note: What is common today, that after washing the head, one first completely washes the right side and then the left side, although it has not been mentioned in an explicit narration, it has been said that, firstly, it does not contradict the two mentioned narrations, and secondly, it is in line with precaution. God willing.