Cut a circle out of the polystyrene and make a hole in its centre (the hole should be a little smaller than the plastic container).
Cut a length of plastic pipe slightly longer than the height of the planter. Cut out a semicircle at the edge of the polystyrene for the pipe to fit into.
Poke holes in the base of your plastic container and place it under the hole in the centre of the polystyrene. Water will seep through the holes from below.
Place the plastic container and polystyrene disc with plastic pipe into the planter. Fill the planter (including the small container) with soil, ready to sow your seeds.
Pour water down the pipe into the basin beneath the polystyrene. The roots of your plants will drink up the water as it seeps from the plastic container up into the soil.