Step #1: Gather your materials. Everything can be purchased at a nursery. You'll need pebbles. (I foraged for mine at the park instead of buying them.) When in doubt, always get more pebbles than you think, especially if you want to have a big layer showing, like I do below. Next you'll need charcoal. I promise you won't get the black lung from your newly purchased charcoal. This stuff is so important to a happy terrarium. It will absorb water runoff so your plant doesn't get moldy. Gross. And then die. Sad. Get potting soil too. Nothing fancy here, it's just good if it has a little fertilizer in it. Pick your plant. You'll want something that likes warm, moist climates. Ferns, moss, and begonias work really well. You also want to select something that will fit your vessel. Maybe you have a really wide base and can fit three or five little plants, or maybe tall and narrow is the way to go. Lastly, find a vessel. A true terrarium has a lid and becomes it's own little ecosystem inside. However, you can cheat and go lidless if you want.
Step #2: Mix the charcoal with the pebbles. You want the ratio to be about three parts pebbles to one part charcoal. Fill the bottom of your terrarium with the mixture. You want the bottom layer to be at least an inch deep.
Step #3: Take your plant out of its container by squeezing the sides and wiggling it out. Don't be scared that you're going to break your plant here. I separated off a section of the plant below because it was just too big to fit. Place the plant directly on top of the pebble/charcoal mixture. Next, have your potting soil ready to go in a mason jar (trust me it's so much easier to maneuver when it's in a jar!) With one hand protecting your plant, pour the soil over the roots. Take breaks from pouring to gently pack the soil down. Pour enough to cover the roots and provide support for the plant. Once your plant is in place lightly mist with water, and add any doodads you have laying around.
If you were to make a terrarium for succulents, just use sand instead of dirt, and you definitely want a vessel that is uncovered.
Lightly mist your terrarium about once a week, maybe less if it's covered. Just keep an eye on it. If the soil is moist and the leaves are bright and shiny, you don't need to water. But if things are drying out and looking wilty, then a little misting might be in order. If your plant starts to outgrow its container, either repot it, or trim it back.