Add a thin layer of pebbles or gravel to the bottom of your terrarium.
Add potting soil to your terrarium until it reaches about halfway up the height of your terrarium.
Plant your cacti or succulents into the potting soil. You can add one plant or a few in each terrarium.
You can cover the top of the soil with decorative pebbles for an added stylistic touch.
Store the terrarium in an area that receives a moderate amount of sunlight. If your plants begin looking washed out or faded, then they're receiving too much light.
Check the water level by sticking your fingers into the soil every day or so. If the soil is very dry, add a little water. If the soil feels moist to the touch, leave the plants alone for another day.
Add one part fertilizer to four parts water in a watering can. Mix the solution. Water your plants with this instead of plain water once per week.