Timing contractions help!

Brianna

This is my second but with my first my water broke and I never really felt any contractions, I’m currently 36 weeks 3 days went to the doctor yesterday and was dilated to 3cm. After that I felt back pains and period like cramps but as of today it’s more back pain and what I feel like to be contractions. I’m trying to time them and I assumed it was from start to finish But I read online from start of one till the start of another? I’m using an app to track them, it would be helpful if someone could clarify this for me please and thank you!