Hi Rebekah,
You won’t want to bring the water to a boil with the herbs in it, that will cook off a lot of the medicine. The poultice doesn’t necessarily have to be hot, although that might feel good. You could try just getting the poultice wet, put that on the ankle, then wrap the bandage around it. The allantoin in the comfrey will soak through the skin and work on healing the ankle.