It's a lot easier than you may think!
First, make sure that you have all the required materials for building your terrarium. The most important of these, of course, is the terrarium itself. Virtually any type of clear plastic or glass container will work just fine. Bottles, fish bowls, jars, you name it. There are even special containers built just for terrariums. Your choices are endless; just make sure that you choose a clear container that's been cleaned with hot soapy water, as tinted or cloudy glass or plastic will interfere with plant growth by reducing light.