Most of this post is best suited for consideration when choosing a daily uniform/dress code, rather than one for scenes or events, which might need different considerations. This post is written as if talking to a D/M-type for simplicity.
Starting off, consider: what level of detail do you want to go into? Down to hex color codes, or “something blueish”? A specific dress, or “no pants”? Some details may be more important to you than others. Maybe polo shirts are really important, but hairstyle couldn’t matter less. Are you looking more to lay things out in terms of what’s okay, or what’s not okay?
Consider things that are more the surrounding rules than the uniform itself. What can be done without approval in the moment? What always needs approval? Are you planning on the occasional exception? Is it important to keep the uniform neat-looking?
Now consider the categories of clothes the s-type may have reason to wear. For example: daily, formalwear, activewear, swimwear, pajamas. You’ll want to make sure those are covered in some way. Do they have a job or any other obligations that require specific clothing—how can you work with that?
Within those, consider each item of clothing to be addressed. For example: shirts/tops, pants/bottoms, socks, underwear, bras, shoes, jackets. And accessories—can they be worn, which ones, when, and where on the body? If they wear any mark of ownership, such as a collar, that’s important to note. Are there any “features” that are important—having pockets, being waterproof?
Now think beyond clothes. Shaving rules? What about their nails? Makeup—if allowed, when, and what kind? Daily hairstyle? A step up: body mods. Piercings—if allowed, where, and what kind of jewelry? Tattoos—if allowed, where, and of what? Hair dye—what color? Cutting their hair—what style?
Consider some general concepts. Is it important that they look “submissive”—and what does that look like? Would you rather skew a little modest, or a little not? What do you yourself wear—and how do you want their sartorial choices to compare? Do you want a more plain everyday look, or something that stands out? Where do you want to fall on the casual/formal scale? If you’re weighing in the s-type’s opinion—what do they think of your ideas?
Now some practical matters. Are you okay with the vanilla world seeing this uniform, if they will? Is the uniform practical for what the s-type does with their time? Is it comfortable, if that’s important? How easily/quickly can you acquire it in the s-type’s size—now, and likely in the future? What amounts of money are involved, and who’s responsible? How does it fit the weather they’ll encounter—if there are big shifts between seasons, do you want the uniform to shift with it?
For reference, how my slave uniform currently works. A specific shirt, and specific black leggings. For the rest of my daily uniform, I wear (specific) black knee socks, black bra, black underwear, and my smart watch (kept charged, and worn on my left wrist). Plus, my collar and my wedding ring (left ring finger or pinned to my leggings to not lose it during certain tasks), which are basically always on and basically not considered clothes. My hair is to be left down. Shoes, if I wear them, are one specific pair of black sneakers. I’m allowed to choose my own jackets. I sleep naked. Other notes we have are that my uniform should look nice, clean, and of an appropriate size. Otherwise, I am to shave any body hair whenever I shower (regularly, and have my work inspected after in a specific position), keep my bangs at reasonable length, and keep my nails short. I’m allowed to add, remove, or change out uniform clothing items without prior permission if it is necessary to maintain a vanilla facade (say, asked to try something on by a vanilla family member), as long as I notify Mistress of it as soon as reasonably possible. However, I am to ask permission before changing underwear if doing so due to soiling via arousal (or message Mistress notification of it if she’s unavailable). A few practical items like a bathrobe, apron, etc., are pre approved.
Anything and everything else is to be approved each time.