The title says it all 
Are you indicating you want to self-study, or are you asking about general preparation for the exam? Honestly, it depends on you. Do you already grasp the material/concepts well? For general prep, it is always good to start preparing a month in advance, use a study book, watch YouTube videos, take practice exams from the College Board. But really, it all comes down to you and your situation. I’ve never self-studied for an AP exam before, but in terms of Micro, which I did take, you really need to have a good understanding of how to draw and label the different types of graphs, answer questions about supply and demand etc. etc. Try ACDC Econ on YouTube if you’re self-studying, use 5 steps to a 5, whatever you feel works best.