Soil should be loose, well drained and sterile. Do not use common garden soil because it will soon pack together. For tropical terrariums with foliage plants, use one part soil, one part peat moss, and one part perlite. Or, use a similar soil mix. If you want to create a cactus garden, use a coarse soil made of one part soil and one part sand.
All ingredients are available at floral shops and garden centers.
Place plants in the terrarium, and cover roots with soil. Use rock, driftwood or other materials to add interest. Arrange your plants outside the terrarium before you plant them. For terrariums viewed on all sides, put larger plants in the middle and smaller ones around the edge. For those you view from one side, slant the soil and plant tall plants in back, with shorter plants in front.