I drink a lot of strong hibiscus "tea" - I use quotes because it's really not tea at all, just a hot beverage steeped with dried hibiscus flowers. I have no idea how fresh petals would work. If this were me, I'd experiment with your plants using simple hot water first.
For a drink using dried petals, I'll use about 1/4 cup by volume of dried petals in a 10-12 oz mug. Steep in hot water for 4-5 minutes using a tea filter or a French press so you can separate the liquid. It will be quite red and very tart, but also a little sweet. You may already realize this since you compared the flavor to cranberry.
You can also make a cold tea by steeping dried petals in a larger volume of water in the fridge overnight.
I would experiment with these options to determine how to approach quantity, temperature, and time variables when making your beer.