Spread it on top of a pot of very wet peat. Keep it wet to the point of waterlogged. Keep it under bright light, equivalent to the brightest shade that won't cast a shadow; prolonged, direct sunlight can burn it.
I keep mine in a 10x20 greenhouse flat with a clear dome.
Scot forgot one very important part of the equation. you have to completely forget about it for while. if yah watch it, it wont grow. another way to get lots of it is if you dont want it. i can generally grow it in pots that i REALLY dont want it in but getting it to grow in the pots i want it to is difficult
It can be mysterious stuff. I've potted a few Sarrs at the same time in the same mix and "seeded" with live sphagnum from the same source. The pots get the same light and the same water and everything is as same as same can be. Then the sphagnum spreads like a weed in one pot and not at all in another.
That's quite possibly the golden rule of growing Sphagnum. If you have something small that can be drowned out- it will. If you would like it to cover a large area around a hardy plant, it will be ornery. Oh well, I figure it's better to trim pots where I don't want it and transplant it elsewhere than to not have any at all. As it stands, I'm very limited in sphagnum, looking for somewhere to get more, don't really wanna buy a brick but I'll probably end up doing that.
I've been using plastic trays, with a layer of peat and sand underneath the live LFS. I keep it wet and at a SW window sill. Seems to work. I take bits of it and put in all of my pots and they spread slowly.