They are pretty easy to grow from seed and once you have some they are easy to increase by division. The main thing to remember is when sowing from seed, the temperature must not exceed 18°C. Sow the seed in a tray on top of the compost and aim for a temperature of around 15 to 18C and they should start to germinate in two to three weeks. Once the seeds start to germinate cover with a thin layer of Vermiculite as this will help the seed to root in to the compost. You can sow them from March to September planting out from September to October to flower March to May the following year.
Polyanthus are a hardy perennial so will grow for many years and more plants can be obtained by splitting the plants in Spring or Autumn. Just lift the plants and wash off the soil so you can see the rooted off shoots and then just take a sharp knife and cut away the off shoots and plant on as a plant in its own right. They will thrive in any garden soil but prefer one that is rich and moist. If possible they also like some shade although the ones I have in are in full sun at the moment but these will be moved for the summer to somewhere a little cooler.