Hi, and welcome to g o y, there are about 700 species in this genus, the majority of them being tender, some need a minimum winter temp of 41-45deg f, others need a min of 51-59, so it would help if you knew the species.
Grow them in a greenhouse,{or indoors}in 2 parts john innes no2 to 1 part leafmould, in bright filtered, or indirect sunlight, at the temperature appropriate to the species you have, Derek.

Hi, Firstly, there is no such thing as an indoor plant. ALL plants grow outside, that's their natural habitat and that's where they prefer to be. I'm not trying to be clever or anything in saying this but I think we all miss this point.

However, there are certain plants that that we would like to grow and we are restricted because of our climate, selaginella being one of them.

Most of the selaginella species are tropical tender plants so I would tend to grow it in a pot, keep it outdoors for the summer months and move it indoors as Derek said for the winter. If you have a conservatory then this would be ideal. If not, then I would keep it in a sunny indoor position over winter.