What is the correct method for performing ghusl (ritual bath) after janabah (state of ritual impurity), especially washing the head? Should it be by rubbing with the fingernails or with the palm of the hand and fingers, in order to overcome whispers of insufficient purification?

Rubbing the body during ghusl from janabah (major ritual impurity) is sunnah (recommended) and not obligatory according to the majority of scholars. The flow of water over the limbs is sufficient for both ablution (wudu) and ghusl. Certainty in the water covering the entire body is not a prerequisite; rather, a strong presumption is sufficient, because requiring certainty would entail hardship and difficulty. This is the stronger opinion in terms of evidence and is a remedy for those afflicted by obsessive doubts (waswasah).