when to start my BC

So I went to the dermatologist today and we decided to try a birth control to help my acne. I'm in the middle of my cycle (like day15 I think) and i dont know when to take it... should I wait until my period??? If I do that I won't have enough of my other pills to make it and he said he wanted me to take both til I finished them. The BC I'm on is Tri-Linyah. It only gives a Sunday start and 1 day start.. 😅