New to BBT. help!

Morgan • Married 💍 Mama🥰 Waylon James💙 Wynonna May💗
Just got a BBT thermometer today and plan to start using tomorrow morning. My periods are ridiculously irregular and I range from 40-100 day cycles depending on what my body feels like doing 🙄 No PCOS. So I would just like some tips that will help me find my fertile window since I never know. OPKs barely work for me so I decided to give this a try. Any do's or dont's?