The first step is to get an idea. This is not all that hard to do. The tricky part is to get a good idea. The way you do this is to come up with lots and lots of ideas and throw out all the ones that aren't good.Read the whole thing.
But where to get ideas, that's the question. Most graduate students are convinced that the way you get ideas is to read journal articles. But in my experience journals really aren't a very good source of original ideas. You can get lots of things from journal articles|technique, insight, even truth. But most of the time you will only get someone else's ideas. True, they may leave a few loose ends lying around that you can pick up on, but the reason they are loose is probably that the author thought about them a while and couldn't figure out what to do with them or decided they were too tedious to bother with -- which means that it is likely that you will and yourself in the same situation.
My suggestion is rather different: I think that you should look for your ideas outside the academic journals|in newspapers, in magazines, in conversations, and in TV and radio programs. When you read the newspaper, look for the articles about economics ...and then look at the ones that aren't about economics, because lots of the time they end up being about economics too.
One of my econ profs pointed me towards this article a couple days ago. Highly recommended! Especially for my fellow PhD students.
Also recommended is Daniel Hamermmesh's The Young Economists Guide to Professional Etiquette [PDF]. It has some good thoughts about what to expect when trying to get something published in an academic journal.