Using a large spoon, or your fingers, dig a hole in the potting mix.
Place your plant in the hole and gently pat the soil around it. If your terrarium has a narrow neck that you can’t fit your hand into, you can use cooking chopsticks, tongs or long tweezers to place your plants and pat them in.
You do want to make sure that there are no air pockets between the roots of your terrarium plants and the soil.
A good trick, which I learned from Tovah Martin's fabulous book, The New Terrarium, she suggests that you put a cork on the end of a skewer or chopstick to tamp down soil, particularly for a terrarium with a small opening.