Here you have the instructions to write an opinion essay by teacher Cristina Cabal (click on the link to visit her blog. It's worth seeing)
Have a look atDulce's blog for some examples.
And why don't you try doing this exercise(Taken from Successful Writing Intermediate)
As I told you in class next week we're going to look at health vocabulary. The presentation gives you some explanations and exercises you can do to help you (by Dulce Rosales)