Grammarly
Grammarly Blog: Parentheses and Brackets
Rules and examples for using parentheses and brackets correctly in sentences.
Get It Write
Get It Write: Punctuating With Parentheses
Here's a great tutorial that explains when to put commas inside or outside the parentheses. A brief quiz checks your understanding.
Other
Grammar Book: Parentheses and Brackets
Included on this site are three rules for correct usage of parentheses, along with examples for all of the rules, and a quiz.