First of all, one important thing to bear in mind is that anyone attempting to put together an outfit of any sort needs to rely on their sense of taste and style. Taste, of course, can be a rather subjective concept, and if one feels that one’s own sense isn’t up to par, it’s advisable to involve someone else in the proceedings if for nothing else than a second opinion. Indeed, even if you normally feel your fashion abilities surpass those of everyone around you, it’s still advisable to ask someone else what they think before you go traipsing out the door wearing some bizarre concoction of clothes. (Note: This rule is negated if the central conceit of your outfit is of a grand enough scale. Or if you are Lady Gaga.)
Now, on to the actual designing of an outfit. When I begin the process of coming up with some new Steampunk get up I always start off with some sort of inspiration. As per example, the idea for the ensemble I wear during my Steampunk makeup and clothing tutorial on my youtube channel (http://www.youtube.com/user/yankee999?feature=mhee#p/u/3/BhaS9VcE560) was derived from the character Grell a la the manga Black Butler. I started by pinpointing Grell’s central color scheme of black, white, and bright red (black, white, and red is also arguably Sebastian’s color scheme, but his red is more a shade of maroon or claret while Grell’s is more in line with the color of blood or roses.) From there I assessed what clothes I had that fit that schematic, as well as the general visual tone of the manga (clean lines, minimal visual foofrah) and from there weeded out any excessively lacey or gauzy pieces of clothing. What was left was a pinstriped skirt made from black suiting, a black blouse, black-red-and-white corset, black boots, and black bustle coat. I assembled this, then added a silver pendant strung on a red ribbon, garnet earrings, and some makeup. Ta-dah! Outfit complete.
This same process can be followed by anyone. Pick a starting point for inspiration, assess the color and style of the inspiration, find clothes + jewelry + makeup that match that style, then put it all together and out the door you go.