It starts by unpacking the lilies you just bought / received. Make sure the lilies tops are pretty equal to eachother in length, and if necessary, cut the stems in the same length. Before putting the lily in a vase, you remove the part leaves which may hang in the vase. The underside should have a piece of about 5 cm oblique to be cut.
The most common error is that people cut lilies on a very small piece (about one cm) diagonally cut. An oblique cut at the stem is better for nutrients to itself and will also be good longer in the vase. It is important to make sure that you're cutting your stems correctly, as whether you're a constantly-monitoring sort of gardener, or someone who likes to leave the vase as an ornament and play some without worrying about the lilies inside, this can make all the difference.
Set the lilies always in sufficient lukewarm water and in a half sunny place in the living room for the longest pleasure.