Base Layer/Drainage layer: Pebbles, Coarse Sand, Leca Balls, Activated Charcoal, etc. 

Separator/Moisture retaining layer: Dried moss

Plantation layer: Soil mix

The soil mix needed to make a closed terrarium should be chunky, aerated and well draining. The roots should get sufficient moisture and at the same time, enough aeration. 

A mix in the following ratio:

1 Cup perlite or lava rocks or pumice or vermiculite,

1 Cup cocopeat,

1/4 Cup charcoal,

1/4 Cup vermicompost

The plants that will live in the closed terrarium will be living under a high humidity environment. While choosing plants, it is important to do your research and choose plants that enjoy high humidity. Ferns, Syngoniums, Fittonias, Creeping fig, etc. do well under the environment that a closed terrarium provides.

A closed terrarium, when bought home initially will need monitoring for a few days- till it finds the perfect spot to settle. This means that it is important to ensure that your terrarium is getting the right amount of light that it needs. Also observe the condensation of the walls- too much condensation means an overwatered terrarium and no condensation means it needs water. 

Plants wilting and turning mushy brown is a sign that your terrarium either needs more light or needs to aired out to reduce the amount of water that it has.