Read some good stuff. It can be any good book , or an editorial of a newspaper , or maybe any article online. This thing will make you better at writing.
Try writing one atleast one page everyday . For example if you read any editorial then try writing it in your own language.
Buy Urdu to English to Urdu dictionary . Or maybe English to Urdu dictionary too. Its better to download an app on your cellphone. Merriam Websters dictionary is best in my opinion. Whenever you come across any new word. Search it's meaning in dictionary and then there is an option of making it your favourite word. It just means that if you search that word again then you will come to know you read it before. In this way you will be able to make your own word bank. You will realize that after some time every word you come across is a one that you had searched before. It may take a month or two to develop vocabulary. It's a continuous process after that.
Basically English essay and englsih précis, they are the subjects in which you excel on daily basis and with continuity in your efforts ,you get good results. If you are regular in doing these subjects ,maybe just give 30 minutes to essay on daily basis ,and then give 30 minutes to précis and composition section. And try doing the things that are in syllabus. Don't go out from that. At first go through past papers , then you will be able to know that which things are required. In précis and composition section you can do five idioms per day and in same way five pair of words per day .
Buy some books for grammar section. There used to be books by Rabia publications. Go through them. Make your basics strong.
Basically more you practice more better you become in thee subjects. These subjects require time on daily basis or maybe on alternative days and they go side by side with your other subjects till exams.
If you stop writing for a month or two , then you will come to know that you have to start from beginning. This is why don't give long breaks. It will be like after a long break ,you will realize that the writing style that you had is now all gone.
For grammar Raymond Murphy's book is amazing. It has good excercises that you have to solve. And by time you will realize that there are less errors in your writings.
Other than this , make sure that you get your writings checked by any good English teacher on regular basis.
For essay paper , make outlines of different topics .search for relevant material like updated facts and examples. Do it like one topic per week. Write and introduction and conclusion. Start by doing these things. Then , after that you will be able to write easily and to expand your outline easily because as I have written before that you will be writing one page on daily basis.
Learn how to make comparisons in essay and how to attempt it. For example it's a different technique for attempting general essay as compared to technical ones. And you can learn these things by attempting some essays yourself and by getting them checked by some good teacher.
Wren and Martin is a good book. I am using Rippons English composition as I feel comfortable with it. But I heard that paper setter gives idioms ,pair of words and all that stuff from the books that are recommended by Fpsc. So try reading them.
You don't need flowery English really. But it must be having less grammar mistakes and with it paper checker must grasp your ideas easily.

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''Sufficient for us is Allah , and He is the best Disposer of affairs" Al-Quran