The IELTS writing paper has two parts:

Task 1

Sample task
http://ielts.org/PDF/120430_academic_writing_%20task_1B.pdf

Task 2

Sample task
http://ielts.org/PDF/120430_academic_writing_task_2B.pdf

When working on your writing for this exam it is important to be aware of how it is assessed. For IELTS there are four criteria:

1. Task response

How comprehensively have you completed the task?

For task 1
– introductory sentence.
– summarise the main patterns and trends.
– compare data or describe changes as appropriate.
– finish with a concluding statement.
– Or describe each stage in a process.

For task 2
Have you followed each of the instructions (usually four)?
– introduce the topic or argument appropriately.
– express and/or discuss a variety of relevant ideas.
– support arguments with examples, give reasons for any opinions.
– finish with a conclusion that summarises what has been said.

– Is your answer within the word count?

2. Coherence and cohesion

– Organise the information or argument logically.

– Write clear, well-ordered paragraphs.
– Use linking words or phrases and referencing devices, such as pronouns or determiners (this, these), effectively.

3. Lexical resource

– Use a range of relevant vocabulary and lexis (phrases, collocations, fixed expressions) accurately and appropriately.
– For task 1, use verb + adverb and adjective + noun collocations to describe trends and changes over time, or to compare data.
– Or, for processes, use a range of suitable vocabulary.
– For task 2, use a range of lexis to discuss the given topic.

4. Grammatical range and accuracy

– Use a range of tenses and grammatical structures accurately and effectively.
– Use prepositions and articles accurately.

Click this link to a series of four British Council videos discussing the criteria:
https://www.youtube.com/playlist?list=PLi58-qf_wIXb6uJBQ64wxY0nuxXo8PVKA

So, to do well, you have to:
– make sure that your general level of English is improving all the time – work on grammar, build your vocabulary.
– develop your writing skills: generating ideas, planning, paragraphing, linking ideas and checking and editing your work to reduce errors.
– get to know the Writing Paper and the two Tasks well.
– practise frequently, learn from your mistakes and build your speed over time until you feel ready to practise under exam conditions.

Another useful link:
https://www.youtube.com/watch?v=-VHgVa4Il40