Leadership development begins with self-awareness - what are your strengths, what are your weaknesses, and how do you learn? So many people want a checklist of what they have to do to advance in a career. Leadership development is about increasing your skills and breadth of abilities. It is not about the next job, it is about every position you will hold in the future. So attending a class or reading a book because someone told you it will help you get a job is not leadership development. It is the adult equivalent of teaching to the test.
If you are really focusing on development, you'll pick out one to two competencies or skills you want to work on for a period of time and learn about them (in a way that is meaningful to you), practice them, reflect on the effect of the changes, adjust accordingly, and start over.
If you don't already know how you learn best, try out a bunch of different methods with little things and see where you are retaining the most. Journal about it at the end of each day and at the end of the week, reflect on what you learned and develop a strategy that will help you learn new skills and competencies in the future.