A daily newspaper
The field soil or clay soil
Slow release fertilizer
A low but large bowl
Take the low, wide bowl as in the picture, the first thing to do is to place sheets of newspaper in the bottom of the bowl so as to block the escape of the the field ground; the papers will degrade over time and leave out the roots.
Now fill the jar with field soil, possibly sieved or at least cleaned up by big stones, it does not need so much to the growth of the plant as to simplify the subsequent operations of repotting and cleaning. Must be avoided as a substrate are peat, compost or other organic matter rich soils.
Take a spoon or tabs of a slow release fertilizer, controlled-release (great one for flowering plants), and mix well to the ground. Do not use the manure that, even if in the soil, in the water would release many nitrogenous substances with a consequent increase in unicellular and filamentous algae (green water).
Now place the rhizome as in the picture, horizontally with respect to the ground and with a 45-degree angle, with the part from which are emerging new leaves pointing upward and the center of the jar and other side that's having to be slightly buried and will point toward the edge of the pot.
In this way, the waterlily, is spread towards the center of the vessel and will have more space to grow.
We're almost done: now we add the ground to the vessel until it covers 2/3 the rhizome. The tip with the young leaves should remain discovery.
Last phase, now cover the soil with a light layer of sand, the sand is only to not disperse the ground when you place your pot of water lilies in the pond. You can then place the pot in the water.