help with OPK!!

Beca

I was lucky enough to get pregnant quickly with my last two but was also only on the BC patch. this time I am coming off 3 years with an IUD and have no clue what to expect. so I thought I would bbt and use OPK's to increase our chance of getting preggo. but is there a best time to use the OPK? How soon after AF do you start using OPK'S?