I read on another site that when a character is thinking something, what they're thinking shouldn't be italic since it isn't the right use of it, but I do this because it's easy for me to remember what's being said out loud and what someone is thinking. If I have been doing the wrong thing, how should I show someone is thinking?
In my manuscripts use of italics means I'm thinking of moving that particular body of text into the dead file. Needless to say I'm not a fan of them. Your words should convey whose thoughts we are in and why. Leaning letters are lazy in all respects.
In my own book, more than one character has an inner voice (though not always their own). This voice is conveyed in italics.
It might not be considered correct, but it helps make clear that there is a voice not normally used by the character in normal dialogue.
I have to agree though that a large block of text is distracting if all italic.
I don't use italics. I use inverted commas around the text. Whenever in doubt, I keep it simple. Have you considered looking through contemporary novels.