Creating a natural dye from organic matter is a lot like making soup stock. To produce the dye, you start by simmering vegetable matter in water for a few hours, and then straining out the liquid with cheese cloth or tulle, a netted fabric. I mordanted this particular batch with alum, and was so happy with the golden hue that it produced.
I can't emphasize how exciting it is to pull out a batch of untreated wool and to see the beautiful golden yellow colour that it produces. Remember that if you're considering doing some natural dyeing, there are a few precautions to take in the process. Even though natural dyes are much less toxic than synthetic dyes, they can still have negative effects if you're mixing up these dye batches in your cooking pots. Designate a separate spatula for dyeing, and I recommend wearing a dust mask.
Also remember that if you're dyeing a batch of raw wool, to keep the batch simmering as opposed to a rumbling boil. Agitating it too much will cause any raw wool to felt.