Plantinga follows Roderick Chisholm in his rejection of epistemological methodism, on the grounds that always requiring criteria for how one knows something leads to a vicious infinite regress, and thus to skepticism. He also follows Chisholm in adopting a particularlist, inductive method of generating criteria of proper basicality. As Plantinga puts it:
"We must assemble examples of beliefs and conditions such that the former are obviously properly basic in the latter, and examples of beliefs and conditions such that the former are obviously not properly basic in the latter. We must then frame hypotheses as to the necessary and sufficient conditions of proper basicality and test these hypotheses by reference to those examples." (Plantinga, Alvin. "Reason and Belief in God", in Faith and Rationality: Reason and Belief in God (U of Notre Dame Press, 1983), p. 76.).
So the idea is that clear cases of particular instances of knowledge are epistemically prior to general criteria for knowledge. From the particular cases, one examines what features they have in common, and then formulates hypotheses to the effect that all beliefs with those features are tokens of knowledge.
I think Plantinga goes wrong by liberalizing and relativizing Chisholmian particularism. Plantinga intends his use of "obviously" in the passage above to be relativized to epistemic communities ("obvious to us folks"), so as to allow controversial beliefs that are nonetheless strongly held in a given epistemic community to qualify as "obvious", and thereby to allow for correspondingly relativized, theism-friendly criteria of proper basicality. This goes against the spirit of Chisholm's approach, as his intent was to only countenance Moorean facts as clear cases of knowledge.
Ironically, Chisholm warns against the dangers of a liberalized standard of clear cases of knowledge in The Problem of the Criterion, the very book Plantinga appeals to as the basis of his fundamental epistemological approach: “We are all acquainted with people who think they know a lot more than in fact they do know. I’m thinking of fanatics, bigots, mystics, and various types of dogmatists.”