Hello saba, 

That's a good question, you have to keep several things in mind while writing answers for descriptive questions. These are as follows:

1)Mention the headings and then write sub headings. This will help examiner have a clear view of your answer script and since they have to check a lot of papers in a day will fetch you extra marks.

2) underline all important points, as sometimes examiner is in a hurry and doesn't have enough time to read all of it so by looking at just underlined part he will surely give you a good score.

3) write according to the marks, don't go on filling pages just for the sake of it. You have limited time use it wisely.

4) practice writing answers before hand, as this will help you build speed.

I hope this helps

All the best