If you have a container that is rectangular, that is better than a normal plant pot shape. Make sure it has holes in the side, so that water can freely reach the roots through the sides as well as the top and bottom.

Some people use pure clay cat litter to pot them, but I also heard that that has given some people problems. I have heavy clay soil, so I used that. Just don't use peat or topsoil or potting soil.

Plant it so that the top of the tuber is very slightly above the top of the clay, on an angle, so that the top is closer to the edge of the pot than the bottom.

If it doesn't have any stems or leaves growing on it yet, submerge it to about 8" first, then as the leaves grow up, move it deeper, to a final depth of about 18".

The link I gave explains it well.

Here is a link that might be useful: Planting water lilies

No one really ever completely agrees on the best ways to pot lilies.
Some swear by using litter, others use nothing put pea gravel. I personally did not have success with those two methods. You CAN use bagged topsoil as long as it is not the black heavy peat type. No potting soil. Home Depot sells a bagged topsoil that is just plain dirt and I have used it for years. Many people just use the dirt in the yards or gardens. You will need to use pea gravel or river pebbles (I prefer the latter) on top of the soil to help keep it in the pot and help keep the lily weighted down until it establishes. You can also buy it bagged at Home Depot, etc. As you put your dirt in the pot of your choice, make a very stiff mud pie with a little water, adding more soil and water and mixing until your pot is nearly full. Position your lily in the pot, pressing it down carefully into the mud, top with a good layer of gravel and rinse it off lightly with the spray nozzle on your hose and then lower it slowly into your pond.
Dish pans, oil drain pans, small concrete mixing tubs, etc. make good pots. I use black dish pans that I get at Walmart.

I use cheap black oil pans, they are flat and wide with no holes.

I use clay garden soil.

I plant the cut end of the rhizome against one side of the pot so they have maximum room to grow across.

river rock on top to cover the dirt and keep fish from digging in it.

Jobes tomato spikes to fertilize.