Here are some interior monologue tips from "Self-Editing for Fiction Writers" by Renni Browne & Dave King:
Never, ever use quotes with your interior monologue. It is not merely poor style; it is, by today's standards, ungrammatical. Thoughts are thoughts, not spoken.
[Using thinker attributions (ie 'he thought', 'she wondered')] should be the rare exception. Whenever you're writing from a single point of view - as you will be ninety percent of the time - you can simply jettison thinker attributions. Your readers know who's doing the thinking.
Unless you are deliberately writing with narrative distance, there is no reason to cast your interior monologue in the first person.
And whether or not you are writing with narrative distance, it's not a good idea to cast all of your interior monologue in italics.
But if italics, first person, or separate paragraphs are to be rarely used, what's the norm? How do you set off your interior monologue when you're writing with narrative intimacy? Quite simply, you don't. One of the signs that you are writing from an intimate point of view is that the line between your descriptions and your interior monologue begins to blur. Readers move effortlessly from seeing the world through your character's eyes to seeing the world through your character's mind and back again.