Once you have, how do you handle your mechanics so that the interior monologue reads smoothly and professionally? As with dialogue mechanics, the sterling value is unobtusiveness. And there is one actual rule, about the only one we will give you: Never, ever use quotes with your interior monologue. It is not merely poor style; it is, by today's standards, ungrammatical. Thoughts are thought, and not spoken.
Also, it's rarely a good idea to have your characters mumble to themselves or speak under their breath:
"Yes, sir, I'll get right on it, sir," he said, then muttered, "soon as I finish lunch."
It might occasionally be possible to get away with this, assuming a sufficiently sullen character in a lightweight story, but it's almost always going to come off as a contrivance.
Other than these two caveats, how you handle your interior monologue depends almost entirely on your narrative distance. Everybody thinks in his or her own words, so your characters' interior monologue is, like their dialogue, always in their voice. To the extent that your narrative is in a different voice, you need to set the interior monologue off, to indicate that it's separate.
When the distinction is sharp, you could use thinker attributions--phrases such as "he thought" or "she wondered." Like speaker attributions, these mechanical tags usually serve to let your readers know who is thinking what. Very nearly the only time you need them is when you're writing from an extremely distant point of view, . . .