How should I use commas with 'not only... but also'?
"Not only... but also" is a correlative conjunction.
Correlative conjunctions always come in pairs and they relate one part of the sentence to the other. "Not only... but also" can be used to connect either nouns or clauses. When it connects nouns, you never use commas, but when connecting clauses that are complete sentences, you use a comma to separate them.
Connecting nouns: no comma
My favorite food is not only lobster but also scallops and shrimp.
Connecting clauses: use a comma
Not only will I eat lobster every chance I get, but I will also eat scallops and shrimp as often as I can.