How do I add an object to the player on spawn?

I want to put two attachments in the local player's HumanoidRootPart and a Trail in the local player (I'm making a shift-to-sprint function that enables a trail behind the player when the sprint is active). How would I do this? I don't want to use a custom playermodel, so would I use a LocalScript in the starterpack?

EDIT: Here's what I've got so far.

game.Players.PlayerAdded:Connect(function(Player)
    Player.CharacterAdded:Connect(function(Character)
        local attach1 = Instance.new("Attachment")
        attach1.Position = Vector3.new(0,2.25,0)
        attach1.Parent = Character.HumanoidRootPart

        local attach2 = Instance.new("Attachment")
        attach2.Position = Vector3.new(0,-2.25,0)
        attach2.Parent = Character.HumanoidRootPart

        local trail = Instance.new("Trail")
        trail.Attachment0 = attach1
        trail.Attachment1 = attach2
        trail.Parent = Character
    end)
end)