Trying for baby #3 struggling need help

So me and my husband are trying for our third but this time it’s not so easy! We have been trying for 4 months and nothing yet. I got my implant taken out 4 months ago and this is the first baby we have actually tried for! It only took me less then 3 months to get pregnant with my son! This month I decided to try ovulation test strips and I have ten of them! When should I start using them. My other fertile app says my ovulation day is gonna be the 17th today was the last day of my period! Iv never taken ovulation test strips so all the help is needed! And any advice would be appreciated what helped y’all get pregnant while trying! Hoping baby will be our Christmas present!