Tips on taking the Clearblue digital ovulation tests?

I'm planning to use these test for the first time this month after being unsuccessful conceiving for many many months. My cycle is very irregular, ranging from 33-39 days so it's hard to know when to start testing. I have 20 tests. Should I start taking them early because I have no clue when I ovulate? I'm tempted to start testing around CD 12/13 and take throughout first cycle to make sure I don't miss it.