Below is the specification from Edexcel for the unit covering beliefs about the Ontological argument.
Ontological — key concepts, strengths and weaknesses.
The specification does not name or identify any particular version of the ontological argument and candidates will be credited with any version(s) relevant to the question. Candidates should show knowledge and understanding of key terms such as a priori and deductive reason and key concepts may include definitions of God, necessary existence.
Candidates should show knowledge of the premises of the argument and key stages in this type of reasoning, drawing on contributions for example from Anselm, Descartes, Malcolm, Penelhum and Plantinga.
Candidates should show an understanding and evaluation of the strengths and weaknesses of the argument and any significant changes it may have undergone, so as to come to an informed judgement about its validity and whether it amounts to a proof. This may include contributions for example from Gaunilo, Kant, Russell, Hick etc.
The Videos below give a very good outline as to the classical argument by Anselm.