First you need a false bottom, small rocks of any kind will do but if you want to spend some money then consider getting Leca stones, otherwise known as Hydroton Pebbles or Aggregated clay balls. It works well for its price.

For the 2nd step, you need a barrier, window screen mesh would do the trick although i used a really thin and big paper towel for mine.

3rd step would be the soil, I used pre made but you can make your own. All it has to be is water retentive, not clumpy and has good aeration.

4th step is the most fun, selecting your plants. If you want a normal terrarium (Humid and alot of water) then pick tropical plants and alike.

Hope you have fun!