TTC after late loss. Need advice please.

Ki
Hi ladies! So, I'm on CD2. My AF started yesterday - 5 weeks and 1 day after losing my baby boy at 18 weeks and 6 days pregnant. My husband and I want to start trying straight away (we fell without trying with Benjamin so I'm not used to all this ttc!). When should I start using OPKs? Like as soon as AF has finished? Or later in my cycle?
I'm also bbt charting, so what should I expect to see over the next few days and around ovulation?
Thank you xx