No more messing around....I'm gettin' serious this cycle!!! Seriously!!!

I went out at lunch today and bought some OPKs...My first time buying these....so hopefully it will be my first and last! I told myself that if I didnt get preg in 3 months after my MC, I would be more serious and use OPKs...so here it goes...I'm on CD3 now...
Questions, Questions, Questions....On what day in my cycle should I start using them??? My last cycle was exactly 28 days long...I dont temp, but I do occasionally check my CM.
HELP!
Thanks!