I started mine off last spring in plastic trays - this was in the greenhouse but they will happily germinate outside when the soil is warm. I made sure they had plenty of water. They didn't all germinate but I had enough to make a viable crop. When they had outgrown the trays I transplanted them in 10" pots standing in a saucer of water like Georgie, in which they thrived. I ended up with four plants in these pots. This was enough to keep us in watercress whenever we wanted it last summer! You can pick as much as you want and it will grow back. I keep them in the shady area by my kitchen. They like shade, they are less likely to flower and this also means the water in the saucer doesn't evaporate so I rarely have to top up the water. You do have to pinch out the growing tips every so often as flowers appear. In the autumn they died back so I put them somewhere out of the way. This spring two out of my four have grown back.
Watercress is one of those crops where shop-bought cannot compare to home-grown. In fact I hardly grow any other cut-and-come-again leaves apart from watercress and chard because there's nothing to beat watercress. It's only slightly more difficult to grow than salad crops and it's quite rewarding to know that you've recreated it's favoured living environment.
