Add Senses and Character Quirks
This step is one I find to be the most important because it's what helps to bring the characters and the world alive. There is a technique I learned in college for adding in senses. It's another step that is quite tedious and long, but it's well worth it in the end. I go through each chapter and as I read, I think of all my senses and what I might be seeing, smelling, hearing, tasting and feeling in each one. I go through each sense in each paragraph as I read through and make notes, so it takes a lot of time.
For instance, if a character is drinking hot chocolate, I can add notes about how warm the mug might feel, how the chocolate tastes, and what the character might see or feel as the steam rises off the mug.
Then I go through again and I add notes for a character's quirks, such as biting their lips or hair when they're nervous or whispering under their breath when they're angry. I try not to use these quirks too often, though, or they just get monotonous. Generally, I won't add them in more than once every few chapters or so and definitely not more than two or three times in each book. It just needs to be in there enough times to show it's a habit.
I make notes as to what the characters are wearing if it's something they wouldn't normally be wearing, and I also make notes to introduce their usual wardrobe and their physical appearance. I like to add these things in such a way that it's part of the dialogue, actions or other characters' thoughts, because adding too much direct description can interrupt the flow of the story. So when I add notes for those details, I do it in parts of the story where it will be convenient for me to do this.
I also add notes for other elements from the characterisation sheets that are relevant to the story or the growth of the character.
After I make all the notes I can think of for the moment, I begin my second round of rewrites.