theres no real specific way to study eco's
but wat worked for me is writing notes - concise notes. Make sure u also use the syllabus so u dont go entirely beyond it wen ur making ur notes. Seriously everything u need is within the scope of Dixon's book, and Bulmer's series too, so use the syllabus, and write wat u think is relevant for each dot point.
Its the same with essays, u just have to know wats being asked, wat info is relevant etc, eg if u have a question on Protection, u'd look through ur text and u'd see that u need to talk about the types of protection, how its implemented, recent trends in protection, how its dealt with around the world, etc etc