Propagating a peace lily is fairly straight forward and achieved through dividing sections of the mother plant. It's really up to you how many plants you would like to divide and grow, although the amount will depend on the size of your plant and how many crowns it has.
Check for crowns: I probably would have preferred to use a plant which had matured more, however, the lily used here has plenty of good crowns to separate. I'm only going to divide this plant into three because it's quite young, but I could separate more.
Remove from pot: Take your mother plant out of it's present pot by leaning the plant to the side and try to keep all the foliage together. If your plant is stuck inside the pot, tapping the side of the pot usually frees it.
Dividing: You can now divide the plant by taking a crown section away from the mother plant by hand gently, or cut sections away with a sharp knife. This lily did not need a knife used and separated easily. The crown needs to have 2 or more leaves and have roots attached to be propagated successfully.
Prepare: You have a number of plants including the mother plant that you will need to prepare for potting up. Check the roots and foliage, and remove any loose parts of the roots or leaves that have brown tips. You are now ready to pot.
Potting up: Four inch pots are a suitable size to use when potting up, and need to be filled with a peat based potting mix (well draining mix). If your soil is already very moist you wont need to water the plant, but if it's dry then water thoroughly.
Aftercare: And, then there was three...Peace lilies enjoy bright light, a good watering and being fed once a month. You do not need to use fertilizer for the next 2 months, though.