TOEFL iBT Integrated Writing Structure.
The basic structure of a integrated writing essay should look like this -

Introductory paragraph
In this set of materials, the reading passage  __(main idea of reading passage)_____, and the listening passage (main idea of listening passage + connection between passages)___ .
Paragraph describing Reading passage
The reading passage __(main idea of reading passage restated)___ .
Main point 1
Supporting details
Main point 2
Supporting details
Main point 3
Supporting details
(The number of main points will depend on the information in the two passages. There is NO standard number.)
Paragraph describing Listening passage
The listening passage __(main idea of listening passage restated)___ .
Main point 1
Supporting details
Main point 2
Supporting details
(How many supporting details you can include depends on time and importance of the details. You DO NOT have to include all supporting details.)
Concluding paragraph or sentence is not necessary
 
Look at the Sahara sample essay  broken down into the same table
Click here to review Sahara reading and listening materials, and sample notes

Introductory paragraph

In this set of materials, the reading passage describes a vast desert area known as the Sahara, and the listening passage informs us that the Sahara was not always a desert.
Body Paragraph 1 describing Reading passage

The reading passage details the characteristics of the Sahara as it is today. This great desert is the world’s largest and approximately the area of the United States. It is a region with a very dry climate where the average rainfall is less than 10 cm and in some parts can go as low as 2cm. In addition to the dry climate, the Sahara has very little surface water. Apart from the River Nile and a very few oases, the surface of the Sahara is arid.
Body Paragraph 2 describing Listening passage

In the listening passage, we are told that the Sahara was a very different place in the past. Around 10,000 years ago, the region was a wet area with lush jungles, grasslands and forests. It was not only able to support large animals such as hippos and giraffes, but also human inhabitants who lived by hunting and fishing and later took up farming. However, sometime around 6,000 years ago the climate changed and the Sahara started to dry out. This process of desertification continued for the next two millennia and by 4,000 years ago the process was complete: the lush green landscape had become the Sahara we know today.
Concluding paragraph or sentence is not necessary

Important Points
1) The words used to introduce the main ideas should be different each time.
Look at the introduction. The writer uses the word 'describes' to introduce the main idea of the reading passage.
What words are used to introduce the main idea of the listening passage?
What words are used to introduce the main ideas in the the two body paragraphs?
2) You should try to use synonyms whenever you can.
Look at the word 'arid' at the end of body paragraph 1. It is a synonym for 'dry'
Can you see any other synonyms?
3) You need linking words to hold your essay together.
Look at the words 'in addition' in body paragraph 1. They tell the reader that you are moving on to another point.
Can you see any other linking words?