Sow the seeds on the surface of acidic soil (50/50 soil/peat moss mix) in a pot. Keep the pot in a bowl containing 1" of water. The best temperature for quick (5-10 days) germination of the seeds is 75-85 degrees F.
When the seedlings are 2-3 inches tall pot them into individual planters. Increase the depth of the water in the bowl gradually as the plants grow. Plant them in their permanent location in early summer.
Cattails should be planted in full sun or partial shade in boggy or wet acidic soil. They can also be planted directly in shallow (up to 6" deep) water. They don't require any fertilization.
Once they become established, Cattails spread rapidly by underground rhizomes and can quickly choke out a pond, so I recommend that you plant them in the pot, with the bottom removed.
Cattails can be propagated by seed or division.