Start with a Waterstone Arabic book that is at your particular level.
Set up an organized schedule to go through the book(s). Make sure that time is set aside for learning the lessons as well as for completing the exercises in the book(s).
Meet with a native speaker tutor periodically while going through the book(s). For the non-native, pronouncing Arabic takes much time and practice. Many Arabic teachers will attest to how learning proper pronunciation helps with learning how to spell and remember vocabulary. Arabic words are spelled exactly how they are pronounced.
Review already completed chapters. It is easy to forget certain aspects of Arabic grammar after studying it if not reviewed. This is also crucial for vocabulary.
Take self tests routinely throughout the course of the book(s). This will strengthen what you have already learned and help you to build up understanding of the language.