Dry those and store them in a dry, dark place. In late March crumble the flower heads over a piece of printer paper. Tiny fine brown seeds will appear, sequester them. In a tray of properly damp seed starting mix sprinkle the seeds like pinches of salt over the surface. Very lightly press them for good seed/soil contact. Use a plastic lid to keep the soil damp. Put the tray in the dark with 50-60 degree temps for a week to 10 days then bring into the light. The seeds should have begun to germinate.
Thin out seedlings when a couple inches tall. Transplant into gardens when about 6 inches tall when the soil has warmed up enough for basil.
I’ve stopped trying to seed them directly because somewhere when the seed naturally falls from the dried flowerhead and lands on the soil surface and finds it spot – is something I haven’t been able to emulate. There is a gravity seed fairy out there somewhere. Same with angelica seed. Good luck!