I did a steampunk costume a few years ago. Was pretty simple. Took a pinstripe suit I had (or get one at goodwill), put a mad-hatter type tophat, some welding goggles around the hat, then a gun-belt with a Nerf-gun. wear some motorcycle boots. I had actually made up some leather spats, but they can be bought as well. If I did it again I would likely change how I did that to a pair of engineer-style boots and tuck my suit pants into the boots.
Round it out with some small detail items, I had a vest which I hung a monocle, then a pocket watch.
It's better if you can get the SWMBO to go along with it and dress up with. Mine did a flowing gown, corset, had a gunbelt with nerf gun and some riding boots. Was a fun thing to do.