Before you throw your creation out into the world, there’s a bit of paperwork you’ll want to prepare first.

Test, test, test

Make sure your mod actually works! Try it with different configurations of mods, too—sometimes you can end up with an accidental dependency. Giving it to other players to see how they feel about it and what bugs they encounter helps too.

Documentation

This also includes screenshots—players want to know what this thing will look like in their game. You can also do things like rendering out all of the parts laid out mies en place if you have a bunch of interlocking stuff. A mix of both illustrative renders and in-game screenshots is the best.

KNES

You don’t need a comprehensive 100-page PDF or wiki for every mod, but many players prefer having some direction on how the mod is used. It also serves as a good reference for players who want to ensure they know what they’re getting (or not getting) before they download. Both of these save you the effort of having to answer the same questions—documentation acts as a good first-line of support for common questions.

How extensive your documentation is is down to each mod. My own personal mod, CryoEngines Extensions, has quite a few parts but is otherwise not doing anything new versus the CryoEngines mod it’s built on top of, so I simply opted to make some pretty infographics of each of the parts and some screenshots. Conversely, parts mods like KNES or BDB are enormous and are geared towards historical recreations and accuracy; as a result they have larger, detailed wikis that are aimed specifically at providing how-to guides on recreating historical spacecraft.

CryoEngines Extensions

BlueDog Design Bureau

Licensing

One of the requirements of posting a mod in KSP is to be explicit about what license you are distributing this mod under. The biggest impact this will have is the ability for others to remix or update your work, especially if you later retire from modding.

While I won’t go into legal specifics, the two things you’ll want to mull over that will inform what licensing structure you will use are:

Longevity. If/when you stop modding KSP in the future, will someone be able to “adopt” your mod and maintain it so that it continues to be playable?
IP protection. If your mod consists of a lot of your hard-laboured creative work (especially if it is a parts or visual effects mod), you may want to consider protecting it so that others cannot use or abuse it, especially for commercial purposes.

Common licenses and examples

Creative Commons BY-NC-ND 4.0: A creative commons license that permits redistribution freely, but prohibits commercial purposes (they can’t sell your work) or derivations (folks modifying and then redistributing your work with modifications). Any distributed copies will need to be properly attributed to you, the author. Example: KNES
Creative Commons BY-NC-SA 4.0: Similar to above, but lifts the restriction on distributing modified work. Others will be able to remix and transform on your work, but with the caveat that they must also distribute their transformations under the same CC BY-NC-SA license. Examples: BlueDog Design Bureau, Rocket Motor Menagerie
MIT + All Rights Reserved: A dual license system, with the highly permissive MIT license being applied to configs and code, and an All Rights Reserved license applying to art assets such as part models, textures, sound effects, and animations. This allows for people to modify and remix any code or configs freely, allowing for community compatibility, but ensures you own all rights to any art assets you have made. Examples: Tantares, Near Future Technologies

There’s no right answer and you’ll kind of have to gauge things yourself. My own personal opinion is towards the MIT + ARR combo—as Beale put it to me, KSP is no longer receiving updates, which makes compatibility longevity less of a concern, and that the quality of mod maintainers can be questionable. Myself, the bulk of my work in my mods is art asset creation, hence the preference for stronger protections. In general, though, you probably won’t go wrong with any of the three above.