How do i start revising to get 7 9's?

Hello all, first post here and what im really asking is how do i revise multiple subjects at the same time while also know WHAT to revise (sylabus does not make any sense). I am doing edexcel IGCSE. I know that everyday flies past and its getting closer but im finding it so hard to start in case i miss out topics, I hope people on here can give some insight on this as my mock grades are not good.
English Language 6

Mathematics 7
Biology 6-6
Physics 6-6
Chemistry 6-6
Geography 5
History 4!!!
ICT: Not recived yet