Actually sit and write out tasks 1 and 2 while practicing. It is very tempting to think of what you would write and not do the actual writing. You will appreciate the importance of using a structured format and avoiding being repetitive only if you practice writing.

Start by reading the How to prepare for IELTS Writing task.

Task 2 carries more marks. Spend more time on it. Twenty minutes on task 1 and 40 minutes on task 2 would be a good balance.

Since task 2 is more important, it may be a good idea to do task 2 first and task 1 later. However, make sure you write for each task in the allotted area since the answer sheet has separate areas designated for each task.

For both writing tasks, it is a good idea to jot down your ideas on the question sheet so that you know the outline of what you will be writing. It may take 2 or 3 minutes but the time spent is worth it.

Writing task 1 requires you to describe a graph / table / diagram in AT LEAST 150 words. I had practiced on a lot of graphs but the task we had was to describe the data in a table! So practice describing all kinds of graphs / tables. See how much of your writing is 150 words. If you write less that 150 words, you lose marks. If you write more, you are likely to make more mistakes. Try and stick to around 150 words.

For task 1, first spend some time looking at the graph / table and understanding the information given. Don't start writing immediately. Make sure you know what each axis of the graph represents and in what units. The following structure is suggested for writing:

A sentence describing what the graph / table shows.

Another sentence describing the broad / important trends shown.

Description of the data. It may not be possible to describe all the data as there may be too much data presented. Describe the relevant and most important parts. If there is more than one graph / chart, describe any comparisons or trends that can be made out.

A concluding sentence which sums up the data / trends.

Practice using a variety of phrases to avoid being repetitive.

The best practice for task 2, which asks you to present an argument, is to read newspaper editorials and magazine articles on current topics. This will help you develop your ideas. A suggested structure for writing is:

Introduce the topic and state your stand, whether you agree or disagree.
Give arguments in support of your viewpoint supported by relevant examples.
State the contrary viewpoint and give reasons why you don't agree with it.
Conclude with a short concluding paragraph.

If there is time left at the end, revise your answers and correct any spelling or grammatical mistakes.