Your first mod

The dead-simple path is extends: start from a vanilla def and override what you like. This is a complete, working mod, a heavier Bison for the late game:

{
  "format": "steel-tide-mod",
  "v": 4,
  "id": "bison-ii",
  "name": [
    "Bison II",
    "野牛 II"
  ],
  "version": "1.0.0",
  "author": "you",
  "description": [
    "A heavier Bison for the late game.",
    "后期用的重型野牛。"
  ],
  "defs": [
    {
      "id": "bison2",
      "extends": "mbt",
      "name": [
        "Bison II",
        "野牛 II"
      ],
      "desc": [
        "Thicker plate, a bigger gun, and a bigger bill.",
        "更厚的装甲、更大的炮，也更贵。"
      ],
      "tier": 3,
      "cost": 520,
      "hp": 1100,
      "speed": 52,
      "requires": [
        "radar"
      ],
      "weapons": [
        {
          "id": "cannon",
          "cls": "cannon",
          "dmg": 90,
          "reload": 2,
          "range": 5,
          "splash": 12,
          "turret": true,
          "muzzleOffset": 23
        }
      ],
      "aiWeight": 2
    }
  ]
}

1. Save that as mod.json inside a folder called bison-ii.
2. In the game, open Settings → Mods and press Open folder… (or zip the folder and press Upload file…). The game says what it loaded, or lists what is wrong, with the path into mod.json for each thing.
3. Start a Conquest match. The Bison II is in the war factory's list from level 2, because that is where the Bison is; it draws with the Bison's art; and the AI fields it too, since the def carries aiWeight.

extends copies everything (art, weapons, cost, where it is built) and each field you name replaces the copy. Leave weapons out to keep the base's guns; write your own list to replace them.

From here, test it in the game, give it art and weapons of its own, and when it plays the way you want, publish it.