You can start using Natural Cycles at any point in your cycle as long as you are no longer using hormonal contraception. For more information regarding when you can start after being on other hormonal contraception read more here: https://help.naturalcycles.com/hc/en-us/articles/360003287094-When-can-I-start-after-being-on-other-contraception-
The most important thing is that you start measuring and that you measure as many days a week as possible. Aim to measure at least 5 days a week to give the app as much information about your cycle as this will help the algorithm get to know you better from the start.