Composition as per my opinion can only be prepare by practice . :p you should have a sense of imagination to draw a scene in your mind to elaborate the given line in the paper .
arrange the main idea in mind, then write keywords on paper, outline would be better, prepare rough idea , give introduction then explain your keywords in four or five paragraphs , at the end conclude it . write in concise English and don't forget to revise.. Words usually given in question but your length of idea should be of 500 to 1000 words .