BBT charting

Amanda • Me 34, DH 38, married 14 yrs. We have a beautiful 13 yr old lil girl & a handsome 11 yr old lil boy! Practicing for #3+!😏😝
So I just went & bought a bbt thermometer & I was wondering if you have to start charting on cd1 or can you start any day? New to the whole bbt charting, so just want to make sure I'm doing it right! Thanks ladies!