How to write psychotic characters and suspense

[Madeline]

I am very excited because I have a new idea fir a novel! However, it is in a genre I have yet to write in, and I was wondering if any of you had tips for writing suspense or thrillers. Also, the main character is going to have two different supernatural spirits fighting in her head throughout the story and this makes her go crazy. What are some ways whichever I could depict or describe that during the story? Thanks!!!