Light: Bright, indirect sunlight. Keep it out of hot, direct sunlight, which will scorch its foliage.
Water: Keep the potting mix moist at all times. Frosty fern will not tolerate soggy soil, which leads to root rot. Use a pot with a drainage hole and empty the drainage tray after watering. Remember to always use room-temperature water for your tropical houseplants because cold water is a shock to them.
Humidity: This tropical native likes moist air. Aim to maintain at least 60% relative humidity around it. It's a good idea to use a humidity gauge near your houseplant, rather than guess. Indoor air can become extremely dry during the winter months without our noticing it. The most efficient way to boost indoor humidity is with a cool-mist room humidifier. Another option is to keep your frosty fern in a Wardian case.
Temperature: Average room temperatures (65-75°F/18-24°C). Try to maintain a steady temp -- Selaginella doesn't like it too cold or too hot, especially drafts. Keep this plant away from entryways and heat/AC vents.
Soil: Fast-draining mix that's slightly acidic to neutral, with a pH range of 5.5 to 6.5. Peat moss-based potting mix with perlite or horticultural sand added for faster drainage is ideal. Selaginella martensii will not tolerate soggy soil.
Size: ~15 cm height, ~10 cm diameter