It may be gray outside, but it's always green in these moss terrariums. We used kitchen canisters, which are readily available and inexpensive; their tight-fitting lids capture the condensation the plants require to thrive.
Moss is all you need for a charming scene -- and it may even sprout appealing "woodland weeds," as in the versions on the far left and far right. But you can add other small plants, such as the orchid, second from left, or the Pilea 'Tiny Tears,' second from right.
For the terrarium, cover the bottom of a jar with horticultural charcoal (doitbest.com), and then put in a layer of potting soil no more than an inch thick. Add plants if you'd like, and then cover the soil with sheet, cushion, or clump moss (have a florist order it for you). Make sure that any leaves, petals, and stems do not touch the glass. Spritz twice with water, and place the lid on the jar.
Keep terrarium in a spot with diffuse light. If too much condensation forms, give the terrarium a little less light or remove the top for two hours.
Start with damp, not wet soil. You shouldn't need to water again. You have created a closed world and the water will recirculate as it evaporates, hits the inside of the lid or sides of the jar, and "rains" back into the soil. If you open the jar for several hours, you might want to mist the plants again to replenish any water that evaporated out of the jar.
So after the first initial setup, when would you water again? Also, any fertilizer needed at some point?