Model essays can be extremely useful because they show you what you are aiming for, and they contain good vocabulary and correct grammar. However, the model essays in some IELTS books seem to confuse students because they are written in too many different ways (so the 'method' isn't clear).
Although there is no 'right way' to write an essay, I think that you should find one way and stick with it. If you read a model essay that is written in a different way, don't worry! Use the essay for ideas and vocabulary, but don't change your method. Maybe you could even rewrite the model essay using your method.