OPK help please?

Steph • UK 🇬🇧 - Married to my wonderful husband for 4 years. Trying for our first 👶🏻 🤰🏼

Hi, so me and my husband are in our first cycle of TTC #1, I'm due AF tomorrow (hoping it doesn't show) I've ordered some OPK that I want to start using if AF does show this month, might sound silly but I haven't got a clue how to use them? So my question for any expert out there is, what day do I start testing and do I just do one a day? Any help would be great for this newbie over here 🙋🏼 thanks and baby dust to everyone 🤞🏻👶🏻👨‍👩‍👧