First of all, you must go to the GRE website to see the list of sample topics for argument critique in order to become familiar with the type of essay that you will have to write in the test.
You cannot write your own opinion in the essay because you have to analyse the argument in an analytical way. Don’t involve your emotions in the essay writing and just analyse the author’s argument in a critical manner.
After reading the writing sample, you should take a note of any information or assumption which is missing. The missing information will show the author’s partiality in the argument that you will have to discuss in your writing in a critical way.
In order to give reference to your argument in the essay, you must find the common reasonable fallacy (for instance, inductive and classic) in the author’s statement. There will always be at least one rational fallacy in the statement which you must find out first.
You should complete your essay before the specified time so as to save some time for editing and reviewing it. You must go through your essay well in order to check its flow and relevance. Besides, you should also correct the spelling or grammatical mistakes (if any) to get good score.