Research your chosen topic and choose a specific argument. The more specific the argument, the better, although you do need to ensure that it is wide enough to appeal to your audience.
Jot down some rough ideas as to how to back up your argument. Essays need to be extremely clear and logical -- every paragraph and idea should relate to the argument. So, if you are writing "third-party technical support teams should not be pushed on people by Internet service providers," you should relate every single idea to this argument. Every idea you put in the essay should add another layer of evidence to this argument.
Write the main points for your essay. Each of these main points will be the topic sentence of a paragraph, and each one should answer the argument.
Jot down a few metaphors that help back up your main point. Think about things that follow the same principle as your main points, but in a different context. Try to write as many as possible -- this will give you more to choose from, which will ensure that you use your strongest metaphors.
Look through your list for metaphors that will relate to your target audience. This is the key purpose of a metaphor -- to take a concept or idea that someone does not directly relate to and apply it to their life in a relevant, pertinent way.
Write a sentence or two of explanation under each main point. This is creating the outline, or "bones" of your essay. So, if your first main point was "modem installation services are inherently unnecessary," you could explain your point by writing "installing a modem is not difficult -- it is little more than a matter of plugging your modem into the wall."
Write an example to drive each paragraph home. This can be an anecdote or an empirical fact. It is good to use a variety of examples, as different examples appeal to different people.
Use metaphors in the explanation and example sections of your paragraphs. You should not use them in every paragraph, as the reader will grow bored of them. You can use them here and there, though. For example, in a paragraph that begins "modem installation services are overpriced for the amount of expertise they provide," you could explain the topic sentence with a metaphor like "they change a lightbulb, but charge you as if they rewired your house." They didn't actually change a lightbulb, but the comparison is the same. This is the advantage of metaphors -- by relating situations to other situations, you make them more accessible. Even if someone has never installed a modem or even used the Internet, they will likely be familiar with light bulbs, and therefore, will be more likely to empathize with your situation.