It’s a design and fashion style that merges (post)modern technology with Victorian sensibility.
There are no hard and fast rules when it comes to steampunk. The safest way is where you start out with a Victorian or Western look and add a few steampunk elements.
Many people believe that women have to show their corsets for the look, but that is simply not the case. While many enjoy the look, an exposed corset does not make the look, though lots of people really enjoy it. (Please see Melanie Schuessler’s Steampunk outfits, they are amazing, and have no exposed corsets.)
What I find makes something look steampunk is the little accessories. They should tell the story of your character at a glance. Think about the sort of steampunk character you want to portray. A Lady, plumber, adventurer, vampire hunter, accountant, airship pirate? What sorts of things would that type of person wear during the Victorian era? What sorts of accessories would that type of person carry? Then consider how to add a steampunk twist to it.
Adding a steampunk twist to something that is modern may be as simple as painting it brass and copper colored, or adding lace depending on what it is and it’s purpose. Making something more steampunk that is Victorian in nature, can be as simple as using it in a different way, or re-purposing it. I hesitate to say “slap a gear on it” but you’ll find that is what plenty of people do. Adding small details to an outfit are really all of what it takes to give something a steampunk feel.
First thing, headwear. The Victorian era is a time where everyone wore hats when they went outside. Add a top hat, bowler, or even just a fascinator (tiny hat or other ornament worn in the hair). It adds a whole lot of attitude to an otherwise simple outfit. To go a step further, add a feather, cameo, lace, or symbol on your hat and it will turn it up a notch.
Jewel tones, metallics and browns are the most popular colors in steampunk fashion, though, the Victorian era was the one that discovered aniline dyes, which made just about any vivid color possible. I only suggest staying away from heavy usage of blacks because then you start looking more like a goth, than a steampunk figure. Arguably, steampunk is heavily influenced by modern gothic designs. There are a lot of Lolita/Steampunk crossovers as well.
While steampunk is based on Victorian aesthetics, if you want to dress as a woman, you do not have to wear a skirt. In the steampunk world, things are equal among the sexes, though still have a wonderful degree of politeness and courtesy. However, if you do want to wear a skirt, you have a LOT of options. Think about what works with your character. Would they wear a long and flowing skirt, or would it be a tighter, A-line skirt for working among large gears? Would they have a bustle, or a drape on the skirt? Tucks in the skirt, or bustling of just about any style skirt, is very popular and a great steampunk look.
Vests are an easy way to add an extra layer for both men and women. They can be as simple or as elaborate as you would like and are a great platform for detail work. Vests should be fairly fitted, or left open. If it’s poorly fitted it will be more obvious on a closed vest than an open one. Canvas or leather are great fabrics for a hard working look, while brocade and suitings are better for a more refined outfit.
If you do decide to wear a corset as outer or underwear, please wear something underneath. A tank top at the very least will make your corset experience so much more comfortable and prevent chafing. I like to use a dress shirt underneath mine. You can even add lace around the collar and cuffs, remove any pockets, and tea dye it for a “old time” feel.
Adding buckles as closures to just about anything, will add to the look. Chains, especially gold, brass and copper ones are great accents and of course exposed clockwork, such as a pocket watch with a glass back. Leather and lace are great fabrics that especially lend themselves to this style.
In the end, the steampunk fashion is almost anything you want it to be. A lot of layers is a good thing. A simple shirt, or one with lots of details and lace, then a vest, bodice or coat, belt (or two), pants or skirt (or two), great shoes, and a fabulous piece of headwear, plus your ray gun, parasol, fan, goggles or mask (steam powered devices produce lots of smoke and soot), and you’re all set to conquer your next adventure!
Check out the photos of people I’ve taken over the years at steampunk events.