Dig up the lily carefully until the bulbs are exposed and gently extract it from the ground. Examine the root and bulb structure - look for natural divisions - this should be very evident because each new bulb will have its own set of roots/foliage. Divide the bulbs at these points with pruning shears. I would only take about one-third of the established plant for the new planting.
Then, follow the planting guidelines in this article: