To make a loadout with C#, call KCMLoadouts.LoadoutManager.AddLoadout and fill out at least the first 2 inputs: loadoutName and icon.

See the "All options explained" page for an explanation of all the properties. Here are a few examples to go off of:

Bare minimum requirements

LoadoutManager.AddLoadout("BareMinimum", TextureHelper.GetImageAsTexture("BareMinimum.png"));

Same items and whatnot as in vanilla

LoadoutManager.AddLoadout
    ("Vanilla", TextureHelper.GetImageAsTexture("Vanilla.png"),
    lowPriorityItem: "Pliers", highPriorityItem: "SquirrelBottle", specialItem: "FishHook",
    bonusCards: ["PeltHare", "PeltHare"], penaltyBonusCards: ["Opossum", "RingWorm"],
    firstNode: "TradePelts", penaltyFirstNode: "");

All options used

LoadoutManager.AddLoadout
    ("AllOptions", TextureHelper.GetImageAsTexture("AllOptions.png"), unlockLevel: 5,
    lowPriorityItem: "SquirrelBottle", highPriorityItem: "SquirrelBottle", specialItem: "SquirrelBottle",
    currentTotemTop: "Bird", currentTotemBottom: "TripleBlood", collectedTotemTops: ["Canine", "Bird"], collectedTotemBottoms: ["Flying"],
    bonusCards: ["PeltGolden", "PeltGolden"], penaltyBonusCards: ["Amoeba", "MantisGod"],
    firstNode: "", penaltyFirstNode: "TotemBattle",
    teeth: 25, startingRegion: "Alpine", boons: ["DoubleDraw", "TutorDraw"]);