I haven't done any this year as I didn't get enough seed but I've come to the conclusion that the best way to propagate it for a small nursery is to line up the eventual pots for sale, with a layer of seed compost on top of each and a thin layer of grit, then drop a tiny pinch of seed onto the centre of each. The seed germinates well and all grows on, or most of it anyway, making a good, tufted plant without the need to prick out anything, the stage at which very many are lost. The plants of both white and blue in my crevice troughs, I simply sprinkle the seed around the mature plants or anywhere and they come up nicely and grow on without disturbance.