My first terrarium was an elaborate landscape, with little figurines holding hands on a park bench among the mossy plants. It convinced me heaven must be a velvety miniature world. But less than a week later: hell. Leaves shriveled, plants died, moss turned brittle. It was heartbreak for me—and for one tiny couple in love. This is how I learned you can't stick any old plants in some dirt in a jar and hope for the best. There's a right way to make a terrarium: