There are two documents related to TOEFL independent writing that are especially important. The first document is the list of example questions offered by the TOEFL centre. Although it is possible that you will receive another question on your exam, it is likely that the question will be included in this document. You should review all the questions contained in this document and if possible, brainstorm as many of the questions as possible. That way you will be familiar with them and ready to write the best essay possible.
Here is a link to the PDF file that contains the example questions from the TOEFL organization.
https://www.ets.org/Media/Tests/TOEFL/pdf/989563wt.pdf
The other document that is very important is the PDF file of sample essays. The essays in this file are not always examples of essays that will get you perfect score. These essays should be reviewed and edited with your teach rand class to learn what kind of mistakes they contain and how you could improve them to write a better essay yourself.
Here is a link to that PDF file with the sample TOEFL independant writing essays.
http://papers.xtremepapers.com/TOEFL/Sample%20Essays%20for%20the%20TOEFL%20Writing%20Test.pdf (this is a large PDF file)