Step 1) Purchase - or find at home - one or more succulents, potting mixture, pebbles, a jar and any little decorations you wish to add.
Step 2) Head off to the plant nursery/aquarium/department store to pick up supplies if necessary.
Step 3) Try not to let toddler run off with supplies. Especially if in middle of large carpark.
Step 4) Once safely at home, spoon potting mixture into jar using a long-armed teaspoon or similar.
Step 5) Shimmy succulent from plastic pot and add to jar, spooning in more potting mixture as needed.
Step 6) Add a little water. Discover that it's against the laws of toddlerhood to only add a LITTLE water. Remove drowning succulent and start again.
Step 7) Sprinkle gravel around the base of succulent then add any larger pebbles and/or decorations.
Step 7) Give up idea of using the little lady decoration that you were planning to put in your own terrarium (the one you were making in tandem with helping the toddler make his) and instead settle on decorating with the toddler's blue plastic dinosaur.
Step 8) Take lots of photos.
Step 9) Face the inevitable consequence of taking lots of photos.
Have a lovely weekend.