The big problem with collecting discarded moss, as it turns out is that, well, it was November in New England when I picked it up. We had already had several frosts, and the moss had gone dormant for the winter.
Thankfully, moss is incredibly tolerant. As in, you can dry it out completely, grind it up, spread it on a growth medium, and still grow new moss. I didn't pick anything nearly that extreme.
Since I was just trying to get the ball rolling and prepare it to go into the Christmas terrariums, I elected to use sunlight, warmth, and water. All of the moss I collected still had some dirt attached, so no need to add any dirt. I also didn't think if be growing it indoors long enough for it to be able to spread (although that's going to be my next project).
If I was planning on growing these indoors any longer than a couple of weeks, I would have grown them on something. As a temporary measure, to rehydrate it, I spread all of the moss in a plastic container. I detest plastic wrap, so I just used the lid to cover it.
During the day, it sits on a table in a room with one north facing window and one west facing one. I leave it uncovered so that it can get indirect light. I cover it at night to keep the water in.
I think things have been going pretty well. At first, I totally overwatered it, which was not good-- the container has poor drainage and I was starting to see a little bit of mold. Now it's doing quite a bit better-- I even have some new pale green growth on the rock cap moss.