In my state's open air museum there is a dyer showing his profession, using woad. You can't extract any blue from it. You soak the fabric in a liquid made from woad, then you let it dry. The oxidation gives a light shade of blue. The more often you repeat this process, the darker the blue will be.
Masking parts of the fabric with a stuff I don't know and washing it out after the process will give you patterns, similar to the use of wax with batik.