It really depends on what template you are using (although most are non brainers and are perfectly fine).
The template described above is mostly fine and you can use the expressions there to help you along. Keep it simple. 4 or 5 paragraphs altogether maximum (5 if you see you have time) including the intro and conclusion. You must get to your conclusion so keep that in mind.
The basic idea is that you start with the intro with a general PARAPHRASE of the conclusion and premises in 1 or 2 sentences- never copy whole phrases from the actual prompt! The last introductory sentence should simply state that the argument is not logically sound for several reasons - you don't need to get into the reasons! Intro is 2 or 3 sentences long max- same with the Conclusion which basically restates in different words that the arguments were flawed and then ends with a final sentence about how the author should take the recommendations into consideration in order to strengthen it.
The Body paragraphs or what we call Flaw paragraphs contain the brunt of your essay; there should be 2 or 3 of them.
Each flaw paragraph should elucidate:
-what is the flawed assumption (explicitly) (1 sentence) - only 1 flaw per paragraph!
-explain that flaw (2 or 3 sentences)
-make a recommendation on how the author should improve his/her argument in order to make it valid (1 or 2 sentences)
Remember that there should be no "i"; no personal opinion should be included- you are attacking the argument.
Take a couple of minutes to figure out your premises and conclusion before writing and hopefully a couple of minutes at the end to proof.
Our students have gotten consistently great scores on these- it is a question of finding the right line of attack, organizing properly, and practicing this a few times to get the hang of it.
Economist GMAT Tutor