It totally depends on you brother. There are some people who do one subject at a time. Then there are others who take one optional and one compulsory subject at once. And , I have seen people doing 5 to 6 subjects alternatively. They just study one subject and then start studying other when they realize that they got bored with first one and basically that's rotation of 5 to 6 subjects in a week not in one day. One of the top position holder did that thing.
Well, at the end it's you who will have to decide it yourself. Start studying. If you get bored or if you feel like you are not able to concentrate then try studying different subject. In this way , you will be able to knwk about your nature.
And as per your question regarding this thing that what should be a sequence for preparation. Then , I think English essay , English
précis and current affairs. They are the ones that you will be doing regularly ,maybe everyday or maybe by skipping a day or two. And for other subjects, start with Allah's name ,that is ,start with Islamiat. And then try to cover it in 15 to 20 days. Maybe at the beginning it will take more than 20 days to cover, because you need some time to make your routine and to adjust yourself with new things and you will be able developing stamina and cognitive skills in all this time. Then once you got that flow,then just keep on moving. You will be doing other subjects in less time in Shaa Allah. And as aspirants are waiting for Fpsc announcement. Then its better to do compulsory subjects till that time. Hopefully there will be no change in grouping of subjects but there were some rumors and I read it too that Ishtar Hussain said things about changing grouping of subject. After advertisement,there will be enough time to do optionals. But try not giving compulsory subjects more than two months ,these subjects include islamiat ,Pak studies and general science and ability ,as you will be doing others side by side till exams. When you are once done with compulsories then don't wait for advertisement,if it's there then fine but if it's not ,then just start studying optionals that you decided as per grouping till now.
By the way there is 90 percent chance that everything will remain same for coming CSS exams, so don't panic. That's just a 10 percent possibility that I wrote here. Because Ishtar Hussain also said that it will take some years to implement reforms fully.