A Will is a legal declaration of how you wish to dispose of your property on your death. In order for it to be valid it must comply with certain requirements including:
It must be in writing, should appoint someone to carry out the instructions (an executor) and dispose of your possessions / property.
It must be signed by the person making the will (the testator), or signed on the testator's behalf in his or her presence and by his or her direction. This must be done in the presence of two witnesses who then sign the will in the presence of the testator.
A witness should not be a beneficiary in the will, nor be married to, or be the civil partner of a beneficiary