Well, how?

Practice! And see if you can get someone to read your writing and give you some feedback. For the structure, you can have a look at this site. There are a number of examples. http://grammar.ccc.commnet.edu/grammar/five_par.htm

Something to remember when writing an essay is to focus on one point. If the reader can only remember one thing coming out of your essay, what do you want them to remember? Everything in your essay should be there to support whatever this point is.