It's a bit of a broad question, but my advice is to open some existing survivor mods in Mod Editor and explore, experiment, and become familiar with the way the game does things in terms of values, items, etc. It's how I first started modding and now I have a whole bunch of them. :) Never modded in any game before this one.

Character mods are also the easiest in the game to do IMO, especially if you're using existing assets for most of them.

One quick tip I'll provide you with: don't use traits, favorite foods or activities, or other things that are provided by other mods directly. Reason being is if you have, for example, the trait of "Master Trumpeter" on your character that was created by another mod, and someone uses your character that doesn't have that mod, it'll cause a black screen crash or an invisible character. If you want to use them, then you need to copy and paste them into the mod as their own object and give it a different ID name so they don't conflict.

If you have specific questions, I'm happy to help.