In IELTS Writing, you must always keep to the topic set. Never try to prepare sections of text before the exam.
Keep to the suggested timing: there are more marks possible for Task 2 than Task 1.
Organize and link your ideas and sentences appropriately, using a wide range of language and showing your ability (in Task 2) to discuss ideas and express opinions.
If you write less than 150 words in Task 1 or less than 250 in Task 2 you will lose marks, but there is no maximum number of words for either.
When you plan your essay, allow plenty of time at the end to check your work.