Steampunk is a sub-genre of science fiction and features steam powered machinery. What makes steampunk very interesting is the futuristic feel set in the past. This feel comes from the setting very often being an alternate timeline of the past or an alternate universe. Most of these stories take place in Victorian England, the wild west, Gothic horror or post apocalyptic worlds. So the first thing to do would be decide where you want to set this story. If you are going to go the AU route you need to think about what in your universe is different from our own, and how that effected the world. If you’re going to go the post-apocalyptic route you have to think about what ended the world of tech as we know it and why steam is now the power source of choice. Once that’s done start plotting out the story as you would any other. The theme comes first and after that it’s no different from any other story. Just remember that anything that came before steam power is either upgraded or existent and anything that existed after steam power is made steam accessible or nonexistent.