I have learned a lot right here on CC. Looking at others designs, reading the tutorials , and sometimes just learning what a technique is called so I can search Youtube for a tutorial.
I have also taken all the Wilton Courses, Cake decorating courses offered by our Community College and our local trade school. I also have taken adult ed.culinary classes and food safety classes though the 4H. I have learned a lot from each of those that I use, and some things that I just didn't care to use in my work.
There are so many different routes you can take to learn to design the way you want to. The courses can serve as a guide, and help with basic technique, but you will make your own designs in the end.