Ovulating vs. Fertile Days

My husband and I are going to try the Shettles method to see if I can get pregnant with a girl for our 2nd baby (even if it doesn’t work I will still be excited for our 2nd boy, but may as well give it a go).

I feel silly for asking this, but it says to try have sex 3-4 days before Ovulation. Would this be 3-4 days before my “fertile window” starts as shown on the app? Or should I use OPK to chart my highest surge and THATS my Ovulation and I should be having sex 3-4 days prior to that.

Any help is very much appreciated. I have always had difficulty when it comes to tracking my ovulating and have always concerned myself more with the fertile window.