I think one of the hardest things in world building is creating costumes that are both cool, and make sense. Unfortunately, in regards to costuming my characters, I've written myself into a corner in "Pocket in the Sea." By accident, I've sort of set a standard that Albion is pretty uncool by not explaining some things in "Pocket" that, in retrospect, should have been addressed. Where we last left the reader is with this knowledge: Ippy has a wool dress uniform that is more or less as snazzy as other military dress uniforms, and then it has a sort of standard ugly mushroom-colored shapeless sack of a service uniform that is called a cassock but may or may not be one in reality. These come with some sort of undershift/petticoat no matter who is wearing them. Our heroes are, for the time being, laughably short on fashion and long on the dorky. Fortunately for me, 'Sparks' is the first book where the utility of the Ippy uniform needs to be explained, so while I have a lot of work to do to get myself out of the corner, I don't need to shoehorn the information in awkwardly.
Lucky for me uniforms aren't plot important. They aren't even all that relevant. I could get away with never mentioning them again and leave the plot hole as it lies. However, I think that is sloppy world-building and, being obsessive compulsive, I have to clean it up. And then straighten up my desk.... fold some laundry (with socks over my hands if I encounter unmentionables. I just can't abide underwear cooties.) and then make sure that the plot hole patch not only completely fills the plot hole but isn't a glaring tumor nor a break in the prose. Once I've confirmed that it does not detract, I have to verify that it adds to the atmosphere, the world. That is the whole point of costuming after all, to add to the narrative by suggesting a time, a place, a lifestyle, a rank, a personality without telling the reader about the character in a block of awkward exposition or superfluous get-to-know the character conversation.
When I review a book, dialogue is what generally adds a star or removes a star. Once in a great while I'll take off more than one star for unbelievably bad dialogue, but I've never given a book more than one star for the sake of dialogue alone, and I've never given a star for good dialogue when the book otherwise fails. The truth is, bad dialogue can destroy a good book, but good dialogue can never shore up an otherwise weak book. So if you assume that a decent book is three stars, then it stands to reason that a decent book with good dialogue is a four star book. Amazon however, has a top rating of five stars, so the last star has to be fulfilled somehow. On rare occasion something is so brilliantly plotted that it actually gets four stars on strength of plot and characterization alone. Dialogue or atmosphere can add the fifth star. But most books need both good dialogue and good atmosphere to get that coveted fifth star from me.
Most of the time when I pick up a book (or game) the emotional atmosphere gets a pass. A lot of writers seem to be good at conveying their characters' emotions in a given scene. Settings, while often generic, are usually consistent with themselves, but costuming is the element of atmosphere that tends to fall apart. There are oodles of costuming tropes waiting to be misused, but it seems like most costuming errors in books fall into one of the following categories:
1. Costume Existence Paradox
Costumes that by no contortion of logic can exist given the reality portrayed in the book. The best example of this would be a character in a decades-have-transpired post-apocalyptic world that is wearing something delicate, flowing and pale that is clean and untorn, but is clearly what they've been wearing for for some time. Or anyone in the same setting wearing rubber domme gear.
2. 'Unremarkable Clothes'
This covers two ideas. One being clothes that would be considered a costume by the other characters that goes unremarked/unexplained. One example of this would be someone in steampunk clothing in a space-age setting. Or someone in modern-esque clothing in a Victorian setting. In both cases, neither character's costuming is ever explained or remarked upon even if it would be natural to do so. The other idea is clothing that is out of place, such as formal clothes at a hockey game that again, goes unremarked or unexplained.
3. Pointlessly Weird Clothes
Characters are costumed like concept runway models, or monks, or eighties j-pop stars. All of them. It's neither cool, nor practical. The clothing described by the author can exist given the reality portrayed in the book, but there's no compelling reason that it should other than The Author Wanted It That Way. (This is more or less the territory I've accidentally gotten myself into. Never fear, it will be resolved.) This is usually compounded with the characters going on about how cool their clothing is, how fashionable they feel, etc, etc. For readers that don't get why it's cool, it's enough to cause excessive eye rolls.
4. Character Clothing Dissonance
This one is pretty obvious -- the character is defined before their clothing is, and when you find out what they are wearing it's pretty obvious that the character would never wear what they are described as wearing. This technically only occurs in books when the character has a choice over their wardrobe, but if a character is in an outfit they would find grating, and they do not remark on it, this can overlap with #2 if it isn't otherwise explained. (E.g. a character that finds their clothing stifling is unaware of other options because of their sheltered existence.)
5. Description Failure
I've seen this rarely in published fiction -- it's far more prevalent in fanfiction -- but when it does show up.... run for the hills! It's pretty much exactly what it says on the tin. Clothing is either described in a very inconsistent manner or it is described as though it's the director's notes for a a movie special effect.
Frankly, costuming errors can be hard to avoid. It's not as simple as do this, not that. Some characters insist on what they are wearing, it's up the the author to spend as much time explaining it as is necessary. Sometimes, the clothes don't make sense until well into the second book of the series when a big reveal occurs. Sometimes the author and the readers just have to bear with the characters until things become clear. At the same time, it's never a bad idea to ask some basic questions and try to sort it out in your own head before asking a reader to do the same. But, these questions are going to change depending on the characters, the book, the atmosphere and the plot of a given book.
I cannot provide a formula, but I can provide an example of the questions I asked my characters to get myself out of the costuming hole I created in Pocket:
1. Why the cassock?
First things first, it's actually not a cassock in the most traditional sense, but it's close enough that it is described as such. Secondly, it's not always used. In the army, it is more typical for a non-augmented paranormal rider to wear the same uniform as the rest of their unit when deployed to make them less of a target. Auggies wear their plug suits or the uniform assigned to their unit as necessary.
The cassock started as a dress uniform that could be thrown on over the plug suits of augments which were considered unbecoming of officers. (Plug suits, being difficult to don and doff, are most often left on when on active duty. Hence, the dress uniform was designed to go over them.) However, it expanded in use to all paranormal riders, even when deployed. (Previously, non-augmented paranomals wore the dress uniform assigned to their unit.) Realizing the utility of the cassock over the wool cloaks previously used by augments in training during Idaho's winter, the long mushroom-colored cassock was introduced as a service uniform for auggies, and then grew in popularity among the ranks.
When society began to realize the potential threat of paranormals during the war with China, pressure was put on Ippy to come up with a 'yellow star' for paranormals. (Since the holocaust never happened in this time line, the practice does not seem half as distasteful as it does to you and I.) The cassock was the swiftest solution to the request.
(It should be noted that the public does not lump auggies and paranormal riders together. While it's common knowledge that some paranormal ability is required to be an auggie due to the Grayscale scandal, auggies are reliably portrayed as inept telepaths or weak psychics that never even use their paranormal abilities, which are, in the public mind, harnessed by the rig.)
The characters are, however, not required to wear it indefinitely. If they are graduated and not required to wear their service uniform, they may go about in whatever they like (At least for now. There are hints that is changing/ sometimes disallowed.) However, as graduation can take a very long time, many people find themselves more comfortable in the cassock than more fashionable clothes and stay in them, hence wearing a cassock is a reliable sign of a paranormal.
2. How did they come up with a cassock, not a trench coat? A trench coat seems so much more likely.
At the time the uniform was designed, late-medieval turkish-styles clothes were very much in vogue, as described in "Corner of a Round Planet." The cassock was a very stylish dress uniform when it was introduced.
3. Fine. Why the gawdawful mushroom color?
The world doesn't have a huge synthetics industry. Hence, not a lot of dyes. The 'gawdawful' mushroom color is the color of unbleached, natural wool.
As it should be, there's an obvious method to the costume madness.