Praise be to Allaah.

Firstly: 

You do not have to follow a particular madhhab; rather you have to ask a scholar whom you trust, one who is famous among the people for his knowledge and virtue, then follow the religious rulings that he explains to you. It does not matter if there are differences of opinion among the scholars concerning religious matters, because this is something that Allaah has willed by His wisdom, and the Muslim who is not able to work out the truth (ijtihaad) is only obliged to ask the scholars, and he does not have to do more than that. 

Secondly: 

In the answer to question number 11497, we have explained how to do wudoo’ (ablution) for minor impurity in detail. Please refer to this question. 

Thirdly: 

With regard to how to do ablution (ghusl) for major impurity, the answer is: 

There are two forms of ghusl: 

Acceptable form: in the sense that if a person only does ghusl in this manner, his ghusl is valid and he becomes purified from major impurity. But if a person this kind of ghusl properly, his ghusl is not valid. 

Complete and preferred form: this is the type which it is mustahabb to do but is not obligatory. 

As for the obligatory and acceptable form it is as follows: 

1.     Form the intention to purify oneself from impurity: janaabah (impurity following sexual discharge), menses or nifaas (postpartum bleeding).

2.     Wash the entire body with water once, making sure that the water reaches the roots of the hair and the places that it cannot reach easily, such as the armpits and backs of the knees, as well as rinsing the mouth and nose, according to the correct scholarly opinion.

Shaykh Ibn ‘Uthaymeen (may Allaah have mercy on him) said in al-Sharh al-Mumti’ (1/423):

The evidence that this ghusl is valid is the words of Allaah (interpretation of the meaning): “If you are in a state of Janaaba (i.e. after a sexual discharge), purify yourselves (bathe your whole body)” [al-Maa’idah 5:6]. Allaah did not mention anything apart from that. If a person washes his entire body once, then it is true to say that he has purified himself. End quote. 

As for the complete form, it is as follows 

1.     Form the intention in the heart to purify oneself from major impurity: janaabah (impurity following sexual discharge), menses or nifaas (postpartum bleeding).

2.     Then say Bismillah and wash the hands three times, then wash any dirt from the private part.

3.     Then do wudoo’ as for prayer, a complete wudoo’.

4.     Then pour water over the head three times, and rub the hair so that the water reaches the roots of the hair.

5.     Then wash the body, making sure that the water reaches all parts, starting with the right side of the body and then the left, and rubbing it with the hands so that the water reaches the entire body. 

The evidence for this mustahabb form of ghusl is: 

It was narrated that ‘Aa’ishah (may Allaah be pleased with her) said: When the Messenger of Allaah (blessings and peace of Allaah be upon him) did ghusl for janaabah, he would wash his hands and do wudoo’ as for prayer, then he would wash himself, then he would run his fingers through his hair, then when he thought that it [the water] had reached his skin, he would pour water over it [the head] three times, then he would wash the rest of his body.

Narrated by al-Bukhaari (248) and Muslim (316).  

And it was narrated that she (may Allaah be pleased with her) said: When the Messenger of Allaah (blessings and peace of Allaah be upon him) did ghusl from janaabah, he would call for something like a vessel used for milking and take water in his palm. He started with the right side of his head, then the left, then he took water in both palms and poured it all over his head.

Narrated by al-Bukhaari (258) and Muslim (318). 

See also the answer to question number 10790. 

One of the important rulings concerning this matter is that ghusl for major impurity takes the place of wudoo’, so the person who has done ghusl, whether the complete or the acceptable form, does not have to repeat wudoo', unless he does something that invalidates purity whilst doing ghusl. 

See the answer to question number 68854.