Place the trapdoor where you want it
Open the trapdoor
Put a solid block above the trapdoor if needed for support
Place a block or ceiling surface above the trapdoor
Attach the lantern to the underside of the block above
Close the trapdoor if desired
If the lantern will not place, make sure you are targeting the bottom face of the block above the trapdoor
If there is no block above, place one first, then hang the lantern from it