The argument for the existence of God that is suggested by the title of this meditation is one you encountered already in part 4 of the Discourse (pp. 19f). It begins here after some preliminary discussion (pp. 69-73) and just after the beginning of the Adam and Tannery page 40, and it continues through the rest of the meditation.
• Descartes’ preliminary remarks contain a number of points that are important, and important not only for the proof. Note at least the following:
• his rule that “everything that I clearly and distinctly perceive is true” (compare this with the first of his rules in part 2 of the Discourse, p. 11, and note his comments on its proof in part 4 of the Discourse, pp. 21f, and in his preface to the Meditations, p. 55 after AT 15),
• when error is possible and what makes it possible (see pp. 71f),
• the distinctions among ideas that are innate, ideas that are “adventitious”—i.e., that we happen upon (in perception, for example)—and ideas produced by us, and
• the distinction between things taught to us by nature and things shown to us by the light of nature.
Together these show what Descartes needs to do to rebuild his beliefs and show the difficulty he faces: all he can be sure of so far is his thought and its content, so he must use this as a basis if he is to provide solid support for a belief in anything else.
• His proof of the existence of God is designed to do just what needs doing: it begins with the content of an idea and concludes the existence of something beyond him. The key step is expressed using the late Scholastic idea of “objective reality” (which was used already in the preface, p. 51).
The objective reality of an idea is the nature of its content, so the idea of a mountain of gold has as its objective reality something very heavy and very expensive even though the idea, as an idea, is neither heavy nor expensive.
There may be no mountain of gold, which is to say that it may not have “actual or formal” reality. The equation of “actual” and “formal” in this phrase is another Scholastic idea: for Aristotle, something’s matter provides its potentialities or possibilties (e.g., what shapes it might take on) while its form constitutes its actuality (e.g., what shape it actually has).
Descartes’ argument then uses a principle asserting that any idea must be traceable to a cause whose formal reality is as great as the objective reality of the idea. My idea of a golden mountain need not be derived from a golden mountain but it must be derived from a source with at least that much complexity of form. He discusses this principle on pp. 73f, but his statement and illustration of it in the synopsis (p. 55 just before AT 15) may be at least as helpful.
Descartes applies the principle in an argument that he presents over several pages (pp. 75-80). You should try to formulate for yourself the steps he takes along the way.