Open the trapdoor so it is in the position you want
Place a solid block next to or behind the trapdoor if needed
Hold the lantern in your hand
Aim at the top surface of the open trapdoor or the block supporting it
Right-click or tap to place the lantern
If it does not place, try attaching the lantern to the block above or beside the trapdoor instead
Close or open the trapdoor again if you need to adjust the lantern’s position