Hello @The Rustic Tart I've yet to paint a cookie, but am looking forward to it.
Julia's video is excellent and covers anything a cookier'd want to know abt painting cookies. There are also many youtube tutorials on the topic. @The Painted Box almost exclusively paints her cookies and they're incredible. She has an excellent video tutorial that shows her method.
Don't give up. Keep working on it. You'll find a solution that works for you. Decorating cookies is just like any art form, everybody has a different way to reach the outcome they're looking for based on a common technique. You'll succeed. You will. Glad you're here