Live sphagnum grows in very pure water and needs high humidity to grow well. If you cannot provide these two, you might as well not bother. But it is not so hard to create these conditions. Here are some ideas.
Growing it in a simple cup or plastic container
This is quite simple. You simply place the live sphagnum you have in a transparent container with a small quantity of water. You can cover the container and open it on and off for ventilation or leave a few holes for ventilation. Change the water in the cup every week or so to prevent the culture from going bad. This method works well for short durations, like keeping your sphagnum alive before you culture it or use it as a top dressing on a pot, etc. Eventually the sphagnum will outgrow the cup.
Growing out the sphagnum
To grow the sphagnum, it needs a little more drainage than a cup offers. You can grow it on soaked dried sphagnum, perlite or a mix of the two or other suitable medium. Some use peat. I have tried using cocopeat, but I find that if it is not really well rinsed, the sphagnum will not grow well and will simply die.
Spread the sphagnum out on the growing medium so that it isn’t too sparse – live sphagnum grows best when it grows in contact with more sphagnum. If you spread the sphagnum too thinly (probably believing that more area covered meaning you will have more sphagnum faster), it will not grow as fast as it will when surrounded by sphagnum.
At the same time, making a thick clump is also not useful, because the sphagnum that does not get light will simply die out with time and not grow. When spreading the sphagnum, I try to create a layer that is 2 strands thick and doesn’t have too many places where a lot of growing medium shows through. It can’t be perfect, but more for a general idea.
Add a hole for drainage just below the level of the sphagnum moss, so that the sphagnum isn’t sitting in the water. You can also simply put holes in the bottom of the pot for the water to run through. This will be healthier for the sphagnum, but you will have to be careful to not let the culture dry out, as all the water will drain out. You can sit this in a water tray.
Growing live sphagnum along with plants
If your growing area has high humidity, you can grow the sphagnum in pots containing other plants. You put the sphagnum on the surface of the pot like you would for creating a culture. The sphagnum actually grows pretty well like that. The only catch is that you have to water or mist it regularly so that it doesn’t dry out.
And it will just grow!