So, the market for urban fantasy is looking pretty saturated - hard to break into. And you've been eyeing up this steampunk thing, but it looks like it might involve work. Fear not! Steampunk is selling, and as a former employee of a large publisher I can exclusively reveal that large publishers don't give a fat rat's for quality, because they make their money on quantity.
And, having read a bunch of the results of this policy, I can now impart to you the never-fail, paint-by-numbers formula by which you, you lazy, talentless hack, can also get a publishing contract (and a legion of diehard fans in funny costumes).
- Setting. In practice you can probably set steampunk anywhere from the Renaissance to about World War II, but its heartland is the Victorian era. Even if you're setting your story in a secondary fantasy world, you should stick in some kind of Victorian reference.
You might think that this will involve research, even if it's only spending a few minutes browsing Wikipedia. Don't worry. Whatever vague impression you have of the Victorian era is fine. Most of your audience won't know any more than you, and they will defend you against any nitpicker who does (or has spent a few minutes browsing Wikipedia) by chorusing, "It's only fiction! Get over yourself!"
- Set dressing. Steampunk is all about the set dressing. No, really, if you get this right you can screw everything else up completely. Memorize these words: Brass. Steam. Gears. Airship. Goggles. Clockwork. Punched cards. Corset. Automaton. Use several of them on every page, and you're golden.
Of these, you would think "steam" was the most important, but actually it's "brass". Brass is shiny, and distracts your readers from the fact that you're a crappy writer. Make everything you can out of brass.
Don't worry in the least about whether making that thing out of brass (or powering it with steam, or clockwork, or using punched cards with it) makes any sense whatsoever.
- Characters. You can just order these from stock. You'll want a square-jawed hero, probably, a plucky gel (that's important), a mad scientist or two, some minions, you know the drill.
Your villain should be so villainously villainous that he hardly has time to plot, between kicking dogs, killing incompetent henchmen and innocent bystanders, and twirling his moustache. He should always seem like he's on the point of tying a girl to some railway tracks while saying, "Aha! My proud beauty!"
Give your main character something they're afraid of, or that they dislike intensely, that makes no difference to their actual behaviour in situations where they encounter it. This establishes their iron will and their unshakeable badassness, and your fans will praise this as "deep characterization".
Be hard on your characters, by the way. There should be a high body count of nameless mooks and bystanders. Beat your main characters up, have them tied up and imprisoned as frequently as possible. Remember: steampunk fans like to dress up in corsets. I trust I don't have to draw you a picture.
- Language. Your characters don't have to talk like a 19th-century newspaper, but some fans will expect it. Don't worry if you don't write this terribly well, nobody expects you to. And it helps to hide the plot holes if your fans are spending all their brainpower on parsing your sentences.
Speaking of which:
- Plot. You do need one, but any pulp plot from the 1930s will do. Some guy wrote a book with all of the pulp plots in, but I can't be bothered to Google for it, so I'm guessing nor can you. Just watch any of the Indiana Jones movies (doesn't matter which, the plot's much the same) and steal that one.
Lots of travelling about in different vehicles (but call them "conveyances") and getting in fights is absolutely essential.
Follow those five simple steps, and steampunk success is yours (or your money back). You can write any old crap, as long as you stick to the formula, and you don't even need to spell or punctuate correctly.
Victoria had an automaton,
Its clockwork was of brass.
And everywhere her airship steamed
The villain kicked her arse.