@Freyrs_storiesBased on a review of a few sources on the internet, there is no one right way, like so much else in writing. One way is using quotation marks, but some note that this can be confusing since that is also the signal for spoken words.
Italics is another option, but, that can be confusing if you also use it for some other type of communication, such as telepathy. Also, it can become difficult to read if there is a lot of it.
You can also just write the thoughts with the same typeface and no punctuation, either with or without dialogue tags:
Joe stalked toward the table where his boss sat. Bastard, he thought. He's not going to get away with it this time.
or
Joe stalked toward the table where his boss sat. Bastard. He's not going to get away with it this time.
To some extent, it depends on what you're writing. For example, if your story has a lot of stream-of-consciousness in it, then the suggestion seems to be no tags, no quotes, and no italics. Otherwise, the preponderance seems to be toward italics, either with or without tags. But, I did find at least one editor who suggested that the modern trend is toward no tags, quotes, or italics for most stories as it "creates less distance."
From personal experience, I would note that if your readers (as I do) tends to use text-to-speech software to listen to stories, then neither italics nor quotation marks to distinguish internal untagged normal conversation. It's therefore important to try to make clear via context when something is an internal thought. To me, that would be clear in the examples I gave above because it is tied to the character's action: "stalked."
I didn't find any sources recommending single-quotes vs. double quotes. I'd simply note that if that is the only distinguishing signal, then it's one that is pretty easy for a reader to miss, especially since it doesn't seem to have wide-spread endorsement.