Choose a notebook. Some writers enjoy writing in cheap spiral notebooks; others are inspired by expensive leather-bound journals. Choose a journal you like so you'll be motivated to use it often.
Write in your workbook every day. By making writing a habit, you will develop your writing ability much faster.
Use writing prompts. Depending on what skill you want to develop, prompts can be a wide variety of things: a question, a list of random words or a plot situation to develop. Prompts are especially valuable for timed writing exercises. For example, when using a question as a prompt, set a timer and write for ten minutes without stopping. Begin by answering the question, but don't hesitate to go off topic. The point of a prompt is not to write a cohesive piece but simply to write without editing yourself. Often you will never do anything further with the pieces you write in response to prompts, but the exercise of writing quickly without trying to write well helps develop your creativity and break through writer's block.
Reflect on your writing process. Write down what you enjoy about writing and what types of writing you find challenging. This will help you know your strengths and weaknesses as a writer, which will enable you to better develop your skills. Once you know your weaknesses as a writer, you can work on exercises that will help improve those skills.
Practice skills to improve your weaknesses as a writer. Once you've identified the skills that challenge you as a writer, use your workbook to develop exercises to practice those skills. If you are poor at writing description, then practice descriptions of random, mundane objects. If you want to develop your ability to write realistic characters, then write characterization exercises.
Change your exercises. While it's helpful to write a response to the same kind of prompt every day, your workbook will be even more valuable to you if you write different types of exercises. Practice your strengths as well as your weaknesses. Try to include many different types of genres and exercises in your workbook.