Generally speaking, Steampunk concerns itself with Victorian characters, themes, and technology level, while taking the technology available and creating devices that, if not impossible, are impractical. The spider vehicle in the film Wild Wild West.
For technology, Steampunk typically cannot use internal combustion engines, nor most forms of "regular" electricity. It is still perfectly acceptable to make use of capacitors or tesla coils, or basically anything found in Dr. Frankenstein's lab. Steam engines, hydraulics, and clockwork are the ways in which to make things. And the more impractical the better. Best example is the device that is the size of a barn that makes ice cubes in Back to the Future Part 3.
Regular propeller airplane? Nope! Must use dirigibles, or bizarre flying contraptions out of Leonardo Da Vinci's sketchbook. Jets are completely out. See the 1961 film Master of the World.
For any device created, it is highly important to have it finely crafted and visually appealing as well as functional. For example, if one were building a rocketship to the moon, it is not simply a matter of putting dials and guages (although there should be as many of these as possible), but there should also be a rather posh and luxurious chair in which to sit while operating the levers and dials on the overly crowded control panel. From the Earth to the Moon by Jules Verne.
Main sources include anything by Jules Verne, a great many things by Edgar Rice Burroughs, and the television series Wild Wild West.
Women and Men have Victorian clothes and Victorian traits. These include romanticism, a high regard for virtue, chivalry, and outright chauvinism. At least for the good guys. The bad guys are really bad and over the top, and usually with some sort of very fancy mustache. The villians of this genre are much like James Bond villains.