For improve your writing skill for IAS exam you should following things :

Read few paragraphs of a book noting down the key words.
Then pause and go through the keywords.

Start Write your notes in your own words using the key words. Here comes the crucial question of the language.

Your reference material might be in English or the language you can choose.

Still your notes and practice answers have to be in the language chosen by you for the UPSC exams.

You can start by writing a single sentence only.

One time you are able to write good sentence in your own words perhaps after few weeks you may start with single paragraphs .

Do not forget the ingredients an interesting introduction in a sentence then logical development of the topic as the main body with a final impressive concluding sentence.

After few months of well written paras build up into a set of interlinked paragraphs that is an essay.