Rule: Use an em dash like a comma, a colon, or parentheses. Em dashes set information apart from the main clauses in sentences.
Examples:
Correct: Sydney needed to sharpen her skates before practice--she hadn't taken care of the blades since last season.
Correct: Rebecca's boat was taking on water--fortunately, with all her years of experience, she knew how to bail.
Practice putting em dashes where they should go in the following sentences:
We wanted to purchase the car because it was dirt cheap come to think of it, the engine was probably rusted out.
Somebody told me that tiger nuts aren't nuts people with allergies can eat them and that they're high in fiber.
It's important to wear the right clothes at that venue they wouldn't even let me enter the space in my sneakers.
Resources for further explanation of using hyphens:
Merriam Webster's How to Use Em Dashes, En Dashes, and Hyphens
The Punctuation Guide's Em Dash