Corsets are definitely something I do not recommend sewing unless you have advanced sewing skills. A really solid corset may be one of your more major purchases in steampunk costuming, although there are plenty of more affordable options.
Historically, corsets were worn under dresses and blouses, not that you would get that impression from any steampunk event. While a few women wear them in this fashion, the vast majority wear them as outer garments rather than underwear.
Corsets are not a requirement. While popular, there are plenty of steampunk costumes that do not involve a corset. Too often I've heard people think they cannot participate because they can't afford, aren't allowed to own, or can't fit into a corset. You can totally be a steampunk without a corset.
Underbusts start under the bustline and extend to a point somewhere
between waist and hip. They are cheaper than overbusts because they require
less material and less measuring. They must be worn with a blouse or dress
unless you plan on getting arrested for indecent exposure.
Overbusts
cover the breasts and torso and end somewhere between waist and hip. Many
overbusts can be worn without a blouse or dress. Just be wary of half-busts
that only rise to about the level of the nipple and deep sweetheart necklines
which plunge between the breasts. T
Fashion corsets are the cheaper corsets. The bones inside it are made out of plastic. That gives the corset some shape but doesn't constrict the wearer. They are generally lighter and less warm than steel-boned corsets, and they do not shrink the waist. You should pick a size that fits your measurements.
If a corset is advertised as having a steel busk, presume it is a fashion corset. If they are only noting the busk (which is the fastener in the front) is steel, presumably the rest of the bones are plastic.
A fashion corset is not necessarily a bad corset. There are plenty of attractive fashion corsets.
Steel-boned corsets will reduce your waist size on average up to four inches, depending how tight you lace it. As such, you usually want to buy one with a waist size about four inches smaller than your natural measurement. (The seller usually specifies how to measure.)
If an online listing for a corset leaves you in doubt as to how to measure or what kind of boning it has, ask by email or phone before ordering. Listings for fashion corsets in particular sometimes get vague on the details.
Stocking suspenders are often offered with corsets so that stockings can be attached to the bottom of the corset. These are generally removable and cause no problem. However, steer clear of corsets with bottom edges that extend into two points front and back for suspenders. These are obvious undergarments, not meant for outer wear. (Here is an example of what to avoid.)
Images provided by The Gentleman's Emporium. Used with permission.