Bought my 1st pack of OPKs today.

💜Jeanette💜 • baby #2 due november 1st😍
So I've been TTC for 8 months too long now. Month after month, big disappointments. I bought my first 7 day pack of OPKs and wondering when is the best day to start testing???? I have about 4 day periods and 35 day cycles. Please help!!!!!