I would say that the best thing you do for both Cambridge economics and oxford economics and management is to focus mainly on super-curriculars, the best way to do this is just to widen your knowledge-base which involves reading books, going to online lectures (ones online by Cambridge, oxford, lse, ucl etc... for example). Another good thing you can do is some essay competitions as of course it's very good to get shortlisted or even win the competition, but the understanding and further apprehension of certain economic theories, concepts can be very helpful in the long-term such as for interviews to display what you know, it also shows your interest to get involved in researching further into a specific economics related areas. There are also some very good videos out there on youtube that can help you find out some other good things you can add to your cambridge/oxford economics appliaction. I guess a summer school can also be helpful maybe but I wouldn't worry too much about this as it isn't really that impactful, you can if you want to though of course. Of course make sure your predicted a-levels or equivalent are also at the best level of grades as that is crucial for any application to oxford/cambridge. In terms of admission tests, I would suggest only starting in the summer as there are only so many past papers for the admissions tests for oxford e&m and cambridge economics and you don't want to get through all of them before you are even close to the test date for the actual admissions test (ECAA/TSA). The best thing I would say to start with right now is ultimately just to start reading economics books as it can give you a very good feel for the subject and it's something that you can come to enjoy with reading more and more books. Hope that helps.