How does Descartes find certainty in the Meditations?

Recall that Descartes’ epistemological goal in the Meditations was to secure a new, solid foundation on which all knowledge could ultimately rest. And he believed that only philosophical inquiry (a proclaimed reliable method) could secure such a foundation. Descartes believed that by assuming the role of a radical skeptic, that is, by rejecting all of his uncertain and possibly false beliefs, he could discover absolute certainty, a single, solid truth on which all knowledge could be built. In essence, the aim of Descartes' methodology is to doubt his way toward certainty.
In a relatively short yet brilliant book entitled Discourse on Method, Descartes articulates his method on how to sort out the kind of beliefs to accept from those beliefs that can be rejected. Although he formulates several rules that would aid him in his quest, Descartes is well-known for one particular rule which states the following: only accept what is clearly and distinctly perceived: do not accept as true anything that can reasonably be doubted.
Descartes’ starting point in the Meditations is his collection of beliefs, many of which he suspects to be uncertain if not false. According to his criterion of truth, he must therefore accept only those beliefs that are “clear and distinct”. Any belief that can reasonably be doubted must be set aside.
It is in virtue of this unique, philosophical method that Descartes is able to "find certainty."

In The Mediations, Descartes determines he can find certainty if he can clear away everything about which he has doubt. What is left standing at the end of this process, he decides, is what he will be certain about. At the end of the process, he determines that the only thing he can be certain about is his own thought: he has what he calls the "clear and distinct perception" that he thinks. From that insight, he comes to the realization that his sense of being comes from his thoughts. As he puts it in a famous formulation, "I think, therefore I am."
The only thing Descartes is certain of is that he thinks. From there, however, he also finds certainty that there is such a thing as a god or Absolute Being, because he determines that he could not think of such a being (which is greater than he is) if it did not first exist.