First of all, you want to choose a suitable jar, or several if you'd like to make a set. You can also use other (closed) containers of your choice.
You need to sterilise your containers before use! I boiled water in a kettle and used that to wash the inside out. This will kill any bacteria that may be inside.
Of course you also want to choose some attractive plants to go IN your jars. Choose something that will fit and that also enjoys damp, moist conditions. I chose some carnivorous bog plants.
Assemble the things you need: soil suitable for the plants you've chosen, gravel (I'm using the aquarium kind, as it's attractive and reasonably fine), and moss.
If you can't buy any moss, get it yourself. I decided the moss at the garden centre was ugly and climbed up on the roof to get my own. Take safety precautions when multi-tasking on a ladder.
Choose the most attractive lumps of moss. Small bits will do, they'll be positioned around your plant later to better store moisture and to improve the appearance.
You'll need some activated carbon or charcoal: I found this box at a pond store, but it wasn't too cheap. It's essential though, as it stops bacteria from growing in your terrarium.
Now let's get started. Add a layer of gravel to the bottom of your jar, the thickness depending on how big your container is. You don't want to take up too much space.
Add a thin layer of activated carbon/charcoal. This will serve as a kind of filter to keep your soil fresh.
Now add some soil. Not too much, as you need to adjust it to fit around the root ball of the plant.
I had so much mud on my hands I couldn't take pictures! The next step is to remove your plant from the pot and shake some dirt off before positioning it in the centre of the jar.
Add more soil around the plant until it sits firmly. This bit gets messy, and is the hardest part to do. (Recommendation: don't use pitcher plants the first time you try this! That was challenging)
Once the plant is in place you want to take your best bits of moss and push them into place around the plant. They'll keep hold of moisture in your container, and make it look less bare.
Take a wet tissue or finger and wipe the inside of the glass to remove dirt. Now water the plants and seal the containers.
The containers will steam up the first few days or so, due to condensation. Leaving the jars shut during this time will allow the mini-cycle to start- water is recycled, as is air and nutrients.
Tada! They'll need to be placed somewhere with sunlight, but not directly, or else they'll cook inside the glass. Occasionally open the containers to air them out or add water if they seem too dry.