To make a loadout with JSON, create a file with the extension .jldt (not .jldr2 or .json!) and ensure you have the 2 required fields: loadoutName and iconPath.

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

{
	"loadoutName": "BareMinimum",
	"iconPath": "BareMinimum.png"
}

Same items and whatnot as in vanilla

{
	"loadoutName": "Vanilla",
	"iconPath": "Vanilla.png"
	
	"lowPriorityItem": "Pliers",
	"highPriorityItem": "SquirrelBottle",
	"specialItem": "FishHook",
	
	"bonusCards": [
		"PeltHare",
		"PeltHare"
	],
	"penaltyBonusCards": [
		"Opossum",
		"RingWorm"
	],
	
	"firstNode": "TradePelts",
	"penaltyFirstNode": ""
}

All options used

{
	"loadoutName": "AllOptions",
	"iconPath": "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"
	]
}