There are two methods of performing Ghusls, both Wajib and Mustahab.
Tartibi (Sequential) Irtimasi (By submerging the whole body).

Obligatory baths (Ghusl) » Tartibi (sequential)
367. In this method, a person should first make a niyyat 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. And if a person, while standing under the water, jerks each of these parts on one’s body with an intention of performing Tartibi Ghusl, it will not be sufficient and the precaution is that one should not content oneself with it. And if a person washes the body before washing the head, either intentionally, or on account of forgetfulness or because of not knowing the rule, Ghusl is void.

Obligatory baths (Ghusl) » Irtimasi (by submerging the whole body)
373. Ghusl by way of Irtimasi is either carried out instantly or gradually. If the Ghusl of Irtimasi is to be done at one instance, then water must reach all parts of the body at one time. However, it is not necessary that the whole body be submerged in water from the very beginning of Ghusl. If a part of the body is outside, and is later submerged with the niyyat of Ghusl, it will be deemed in order.

374. If one wishes to perform Irtimasi Ghusl gradually, then it is necessary that the whole body is out of water before Ghusl commences. Then one would submerge one’s body gradually in water with the intention of Ghusl.