1. Base your plants and how many off the container you use
2. Generally, no fertilizer should be used with your soil medium
3. And provide proper drainage
Keeping these ideas in mind will keep your plants healthy throughout their lifetime.
Note: In most cases, fertilizer or compost should not be used unless you want your plants to outgrow and/or overshadow smaller plants in your terrarium. While fertilizer is great with individual potted plants, little ecosystems like terrariums balance themselves out on their own with minimal care.
Step 1: Gather your supplies
Any size container (preferably glass to be able to see your substrate)
House plants ( or native plants, but they can be picky)
You don't have to use activated charcoal unless you have a sealed container for your Terrarium. If you do, then it's a must. I am using it in this instructable for demonstration purposes. The charcoal helps keep the water clear of buildup of microorganisms that might grow in your substrate, such as algae and keeps the air clean for your plant to breathe. Besides, who wants a smelly plant in their house?
The type of plants you use can be almost anything, but it ALL depends on the container. If you have a plant that will outgrow your container size, or the environment is not one it can grow in (given time, a cactus will rot in a moist closed container), then you'll be kicking yourself later. Do research on what your using beforehand and your plants will be all the healthier for it.
In this instructable I used African Violet, Spike Moss, and dwarf Sword Fern.
For a list of suitable Terrarium plants, follow this link: www.thegardenhelper.com/terrarium~plants.html
Step 2: Layering your substrate
Start pouring your charcoal until it just covers the bottom of the container. Shake it a little to even it out. Then add a couple small handfulls of gravel around the container to make sure you have and even coating around the container. Once you have just covered the charcoal you can now pour slowly from the bag until you have about half an inch of gravel on top. If you just pour straight on top of the charcoal to start, all the gravel will push the charcoal out of its way and you'll have an uneven layer of charcoal at the bottom.
Take some of your moss and just make a thin layer on top of your gravel. No too much though, you are only using it to stop your soil from sinking right into the gravel. Think of it as a screen.
Step 3: Preparing the soil and plants
Now, take you first plant and gently hold loosely onto the base of the stems coming out of the pot. Use your other hand to squeeze the sides of the pot as you turn it slowly while doing so to ease out the plant. The less stressful you are to the plants now the less jarring it is to them to become accustomed to their new environment.
Hold your plant over your container and gently tease out the soil around it's roots. You don't have to get out all the soil but enough to be able to spread the roots out in it's new home. If you have a plant that is "pot bound" (a twisted root ball), teasing may not be enough. You may need to cut into the ball about half way and pull a bit to spread the roots.
Do the same with the rest of the plants you may have and place your plants in an aesthetically pleasing arrangement within the container.
Step 4: Filling in the gaps and adding decor
When it looks good, add in your decor where ever there seems to be spaces in vegetation and is pleasing to the eye.
Step 5: Water your Terrarium
Clean the sides of any remaining dirt, and you have your finished Terrarium!
Just remember to water your Terrarium with your spray bottle and keep it moist but not soaked. If you have a closed container, lift the lid off it for a day every week or so to prevent it being too wet.