Ovulation Kit Confusion

Month 5 TTC and we have decided to try OPKs to make sure we're timing our BD right. I think I'm overthinking the instructions, I'd just like some clarification please.

My average cycle length is 32 days, with a 5 day period. By the chart in my intructions, it says I should start testing for ovulation on cycle day 16. I'm not sure how the cycle days are calculated (silly question, I (know) - is cycle day 1 the first day of your period, or the first day that your period has ended?

I'm using the Forelife in stream tests. Any other advice?

Thanks in advance.