When planting, I did try the bought soils but am reverting to riddled garden soils, wet it to the consistancey of mud pies and add enough to make a layer say 1/2 thick. Flatten that out and then sprinkle some Osmocote fertiliser graunules on top of that. Garden centres sell it as prepack loose granules like barley grains or in pellet form like a bottle cork. B&Q also sell both types. Its a slow release fertiliser even apparently in water. Then add about another inch of mud and then set the cutting on top of that with the crown uppermost. Pack more mud around the cutting but leave the crown just above the level of the mud.
The cutting should go into roughly the centre of the pot or if the crown/s are a one end of the rhyzome place the rhyzome towards the outside of the pot with the crown/s towards the centre of the pot, the rhyzomes tend to grow from around the crown/s. I would then put the lily in a bucket of preferably a trug until it established its self, probably a month but in this weather try shade the bucket without shading the pads, dont be surprised if all or most of the pads die off fairly quickly. Watch the cutting but resist the temptation to poke at it. If in a trug remove any snails you see because with no dead plant matter around they will have only the lily to eat. Also watch for the pads being gradually eaten by something you cannot see, I think there is an 'invisible' midge larvae that eats the pads. If the pads do seem to be being eaten I have been told that a copper sulphate solution will kill all animal life in the water but you would need to do your own research into that. I couldnt get to a garden centre to get copper sulphate so put the 'infected' lily in my plant pond and the tadploes seemed to eat the problem.

Hope that helps