One always reads about and sees pictures of women being laced into corsets, but this was much less common than you might think.
Every corset had fitting laces up the back, but also most had hooks and eyes running up the front. Most women put on a new corset, got someone else to lace it to fit perfectly but comfortably, and thereafter got in and out by the hooks in front.
Now, the extremely fashionable, wanting to skim off every eighth-inch and having maids with nothing better to do, did lace and unlace. They wrapped the corset around the torso, hooked it closed, then the maid (sometimes husband or lover) pulled every bit of slack out of the laces. The corset was so tight that the hooks strained at the eyes, and the lacing was best loosened before these could be moved to unhook.
So you do not step in and out of a corset: you wrap it around yourself like a belt. Also, if necessary, a woman can very well get in and out of one unassisted. The knot on the laces is at the bottom of the corset in back. The woman need only be able to reach around to her tail bone and pick the knot open, then pull a few crosses loose. Alternately, a corset may have one lace from top down to waist, with a second lace below that, but the knots are still all in reach. The difficult part of undressing solo is not the corset but the zillion little buttons up the back of the dress, such as survive on wedding dresses.
The charming and scholarly Janet Burgess is a superb provider of books, shoes, hats, patterns, and dress goods to re-enactors and costumers. She also can't resist using her catalog for Amazon Dry Goods as a forum, as we use this web page. She has dug out reliable evidence that those claimed Victorian 18-inch waists were often much larger.
Young women of the period had a penchant for exaggerating the small size of their corsets, as some do about jeans nowadays. Like jeans, corsets were sold in sizes: 18, 20, 22, etc. You must remember also how short people often were: Queen Victoria was only four foot tall! Such relative midgets might have an 18-inch waist at sometime in their youth.
Jeans have to be zipped (but you don't have to sit down or eat in them). Corsets, on the other hand, can be laced with more or less space between the edges. So if she left a four-inch space, a girl with a 22-inch waist could boast that she wore "an 18-inch corset." Indeed, it seems a six-inch gap was commoner, up to about eight inches.
Lest you inflate waistlines too much, we would like to add the story of a twenty-year-old woman of our acquaintance, 5'2", who would be tall for a Victorian woman, a decent height for many a Victorian man. She had a 24-inch waist by nature, and had never worn a corset or girdle in her life (nor was she anorexic or even given to dieting, nor a smoker or other drug-user, and only got C's in Phys. Ed. -- she just stayed constantly busy and grabbed a hot-fudge sundae whenever she felt like it). At 18, the coming-out age for many 19th century lasses, she remembers she had worn nearly a size smaller clothes, which would have made her waist about 22 inches. So with effort, she might have been much narrower in the waist. Many Victorian finishing schools not only ensured a course of constantly increased tight-lacing for their lucky students, they also kept them on very short rations so that they would not grow unattractively big and robust. With smaller young women, they might have managed an actual 18-inch waist. Such schools also had a certain number of deaths due to "illness" every year: everything was compounded by malnutrition and damage to internal organs from compression.
Photos can lie -- retouching was invented early -- so look very closely at any in which the waist too nearly approaches the neck in size. You will almost always find illogical folds and gathers at the top of the skirt, indicating the real waist was notably larger.
Corsets are an extraordinary experience the first time. You hold your breath while someone straps you in, or maybe you let it out to compress your rib cage. Your buddy says, "All done," and suddenly you have to figure out how to breathe, because all your breathing gear has been immobilized. Tales of being suffocated by a boa constrictor race into your mind.
Victorian doctors and anatomists claimed the difference between men and women was so great that they even breathed differently: men with the abdomen (as you probably are right now), women with the thorax. That, and all the "heaving bosoms" in literature are your clue to survival. You inflate your lungs by lifting and lowering your sternum.
You practice this, and now that you have air you report it to your friends who are along for this experiment. Then you want to see the difference it makes in your figure, and run upstairs at your usual lope to use the full-length mirror.
And just about pass out at the top.
Not all those swooning damsels were faking!
There is a distinct limit to how much air you can gulp in a minute wearing a full corset. Any real exertion in very tight lacing may take you past the oxygen limit. Late Victorian athletic corsets often had elastic panels, so that while your flesh stayed compressed, you could get a little rib action for deeper breaths.
Years of corsetting resulted in the muscles of the torso literally atrophying. The claim that a woman could not stand upright for more than a few minutes without a corset was cruelly true: a woman who had always worn a corset could not. And these corsetted ladies were the only women worthy of the name and of study, to Victorians. Remember this if you have a character who decides to follow dress reform. Many dress reformers (who had gone through the process themselves) advised getting an athletic corset, and then removing the bones or steels one by one, every couple of weeks, allowing the muscles to develop before finally abandoning the cloth compression months later.
So be kind to Victorian heroines in your thoughts. If they are frail and fainting, incapable of physical exertion, they have been made so since childhood by the demands of their society. Give a couple of extra points to the ones who did manage to be athletic in their corsets, too.