Plant your hardy waterlily at the edge of the container with
the crown pointed at the center of the pot. The rhizome portion
should be just buried in the soil with the crown of the plant
at soil level. It may need to be anchored with a rock or something
else heavy but remove the anchor as soon as new roots prevent
the plant from floating out of the soil.
Soil can be anything readily available but avoid potting soils
which contain light materials that will float away. Fertilize
regularly during the growing season with tablets designed for
aquatic plants. Hardy waterlilies rarely need to be repotted
in the growing season unless they ramble out of their pots. They
can be very happy rambling if there is something on the pond
bottom for them to grow in.