I would get to know your body- as the others have said, find out when you ovulate, but at even more of a basic level (and you might know/do this already) take note of the length of each cycle. Don't assume you'll ovulate exactly half way through your cycle. OKPs work for many but it's also worth investing in the time to get to know your own signs of ovulation- cervical mucous patterns primarily but other things too. I'm a huge fan of the website/app Fertility Friend. It's free for the basic usage and it has great online reading tutorials about charting ovulation. And the most Important thing is... Enjoy your partner and enjoy DTD!