What is the correct way to write the Islamic will ? What should it contain?

Praise be to Allah, the Lord of the Worlds; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.

A "Will" means a person's authority to dispose some portion of his wealth after death, i.e. to allocate some of his property for a specific purpose. This includes: (1) The obligatory Will such as paying debt and returning deposits. (2) The recommendable Will, such as allocating a portion of his property for the sake of Allah, and (3) The prohibited Will such as building a Church, Casino or that may harm somebody else.
Therefore, we state, according to the opinion of the majority of Muslim scholars, that the Will is not obligatory. Some of al-Salaf, according to some evidence not accepted by the majority, believe that the will is a must as long as one will leave wealth after his death.
Thus, we say it is desirable as long as it is a form of good deed that brings one nearer to his Lord. Then, there is no specific formula for the will, i.e. any expression that assigns some portion of a person's wealth to another person or organization will be sufficient. So, if one says: 'I give so and so to… by Will after my death, it is sufficient, and this is a form of Will.
A Will cannot violate the rights of inheritance established by Sharia.
Allah knows best.