I've done this by collecting moss from various places in the woods (sunny, shady, to get a variety of species) and whirling it in the blender with buttermilk.
Then, simply paint in, on or drizzle it over the areas where you want to grow moss. It works, but slowly! Gradually the ones that like a particular spot settle in. If you have sunny areas and want the look of moss, you can grow Irish moss (Sagina subulata) and Scotch moss (S. a. 'Aurea'), which aren't mosses at all but flowering plants. They can be purchased in garden centres and both are hardy to zone 4. Various species of spikemoss (Selaginella) have also appeared on the market in recent years and are wonderful accents for the shady garden floor. Weed gently and keep well watered during dry summers.