Firstly, you need to plant them carefully ‼️by starting with putting them in a pot and be careful not to let the clay cover the crown.
Once they started to have young leaves? watch the water level to be the same height as the leaves,don’t let the water flood over the leaves.
After the giant water lily is strong enough ,normally they need to be put in the pond,not a small pot.
So you have to move the pot and place them in the pond instead,but you need to put them in the shallow part first,and keep in mind that the water level is needed to be the same height as the height of the leaves.
After they grow bigger you can relocate them to the deeper level.
You also need to use the good quality clay, not too acid and they are needed to be exposed to sunlight.