Not entirely sure what you mean “formula of understanding.” Are you asking what the general setup Aquinas uses for each article? Or are you asking what are the basic philosophical principles he uses throughout the Summa?
The first one is easy, the second one … not so much.
I would strongly recommend not reading the Summa if you’re not familiar with Aristotle. And I would strongly recommend not reading Aristotle until learning basic philosophical grammatical and logical terminology (as traditionally taught in the “Trivium”). A good book to start out with is The Trivium by Sr. Miriam Joseph.
Those are my suggestions, but there have been some people who have been successful with the Summa in other ways.