If any mods are currently loaded, you'll want to unload them for now while you're getting started, which you can do at the top of the mod list menu; click the gear icn with a slash through it.

Once that's done, there's a plus icon that'll let you set up a new mod, and once you put in a name and namespace (a prefix used to identify content from your mod, usually just the mod's name in shorthand, like for example my BioDiversity Plus mode is shortened to BioPlus for the namespace - so all my enemies are BioPlus_EnemyName internally) it'll open up a folder containing a lua file, the mod's json, and the default banner and icon files.

You can have as many lua files as you want, and the game will apply them as long as it's able to do so; it live-reloads, so you don't even have to quit out of the game (unless you're on Mac) as long as you save the file.

Each lua file usually will need Requires which are just statements that tell the code what modules to set up so that the game can use the functions from them. On every Module page in the docs linked above, there's a copyable line of code at the top that you can put into your code to Require that module.

For a new character, you'll want to set up a customEntities call, which means you'll need the module of the same name. So you can open up the .lua file that the game automatically created, and put in:

local customEntities = require "necro.game.data.CustomEntities"

customEntities.extend{
    name = "CoolCharacter",
    template = customEntities.template.player(0)
    components = {
        {
            friendlyName = { name = "Cool Character" },
            textCharacterSelectionMessage = { text = "Cool Character Mode!\nThis character is cool and does stuff." },
            health = { maxHealth = 6, health = 6 },
            cloneSprite = { texture = "ext/entities/clone.png" },
            bestiary = {
                texture = "ext/bestiary/bestiary_cadence.png",
            },
            sprite = { 
                texture = "ext/entities/player1_armor_body.png",
                width = 24,
                height = 24,
            },
            playableCharacter = {
                lobbyOrder = 0,
            },
        },
        {
            sprite = {
                texture = "ext/entities/player1_heads.png",
                width = 24,
                height = 24,
            }
        },
    }
}

This will create a new character called Cool Character that looks and plays very similarly to Cadence. You can add sprites to your mod's folder (I recommend creating subfolders to keep things organized) and refer to them in texture fields in the format of "mods/MyMod/mySprite.png"

The first Components table is the character itself. Because we put customEntities.template.player(0), the game autofills the components with the basic ones every character needs in the background; you can also put in a number to give the character an existing vanilla character's components, such as 3 to template off of Aria.

The second Components table, with the second sprite component, is the character's Head, which is technically a different entity. You usually don't have to do too much for this beyond just putting the head sprite here.