Help with OPK's please!

​So even though we've been TTC for 19 months I've not really used opk's, only for 2 cycles way back when we first started TTC and even then I never got a proper positive result! Don't think I was using them correctly!!! Really since then we've just been trying regularly during my fertile window! I know I am ovulating, all my blood work is ok and AF is regular! I want to use opk's this month but I'm not really clued up on them...I mean I get the basics but would like to know how to use them for best results, like is there a best time of day to test etc? I'd really appreciate any tips! Today is CD 6, AF is just leaving town! Thanks ladies! X