When a speaker is clarifying something or expounding upon what they say, use dashes on either side, without spaces. A good rule of thumb is the sentence should make sense even if you remove what is inside the dashes.
Correct: The movie—and I hate to admit it—was really good.
Wrong: The movie (and I hate to admit it) was really good.
Dashes should be used at the end of a sentence when a speaker is interrupted.
Correct: I’m Dave and welcome to my channel. Today we—