local Mod = RegisterMod("Your mod name", 1); -- 'Mod' is a placeholder
local game = Game()

-- your item
-- register your item
CollectibleType.COLLECTIBLE_YOUR_ITEM = Isaac.GetItemIdByName("Your item")        -- must be the name you called it in the item.xml file

function Mod:onTearFire(tear)

    local player = Isaac.GetPlayer(0)    -- gets the player (must be modified for coop support)

   player:HasCollectible(TrinketType.YOUR_ITEM) then    -- if the player has your item

        -- make sure it doesn't change other tear like things (may needs to be updated for Repentance)
        if not (tear.Variant == 4)        -- its not Bob Rotten Head
        or not (tear.Variant == 9) then     -- its not Chaos Card

            tear.TearFlags = tear.TearFlags | TearFlags.TEAR_HOMING
            tear.TearFlags = tear.TearFlags | TearFlags.TEAR_SPECTRAL
        end
    end
end
Mod:AddCallback(ModCallbacks.MC_POST_FIRE_TEAR, Mod.onTearFire)