How do you ladies know ?

My periods this year have been irregular , I have ovulation tests (clear blue digital) but I have no idea when to use them! I don't want to use them since there's only about 9 (they're pretty pricey to buy more sticks) . I just don't want to use them and never get a positive and then that's it they're done . I need to stock up before I start to begin using them ... How do you ladies with irregular periods know when to test if you don't know when your period is coming ? 😔