Q. When do you use well or good?
A. In general, use well to describe an activity, good to describe a thing.
When it is an activity being described, use well, as in “He did well in the spelling bee.” Well is an adverb in that instance, describing the verb. When it is a condition or a passive state being described, use good, as in “You’re looking good tonight!” Good is an adjective in this instance, describing the noun.
With feel good/feel well, it is more complicated. In this case, the word well is being used an adjective meaning ‘healthy’ — so it is OK to say, “I feel well.” You can say “I feel good” also, but it is more informal, borderline slangy.