New way of assigning the costume

local Mod = RegisterMod("your mod name", 1)
local game = Game()

TrinketType.TRINKET_YOUR_OWN = Isaac.GetTrinketIdByName("Your Trinket")		-- trinket
Mod.COSTUME_YOUR_COSTUME = Isaac.GetCostumeIdByPath("gfx/characters/your_costume.anm2")	-- costume

-- Option 1 assign the costume for your trinket at the very start of the game. It also works when you restart the game.
function Mod:onNewStart(bool)
	if bool == false 					-- triggers on the start of a complete new game
	and CTC_API ~= nil then				-- check if the 'Trinket Costume' mod is active

		-- CTC_API:MakeTrinketAvailable(trinketID, costume path)
		CTC_API:MakeTrinketAvailable(TrinketType.TRINKET_YOUR_OWN, "gfx/characters/your_costume.anm2")
	end
end
Mod:AddCallback(ModCallbacks.MC_POST_GAME_STARTED, Mod.onNewStart)


-- Option 2 assign the costume for your trinket at the first frame of the game. Also works upon restart.
function Mod:onModUpdate(player)
	if game:GetFrameCount() == 1
	and CTC_API ~= nil then

		-- CTC_API:MakeTrinketAvailable(trinketID, costume path)
		CTC_API:MakeTrinketAvailable(TrinketType.TRINKET_YOUR_OWN, "gfx/characters/your_costume.anm2")
	end
end
Mod:AddCallback(ModCallbacks.MC_POST_PEFFECT_UPDATE, Mod.onModUpdate)