Buy a terrarium from your local pet shop.
Put about 2-3cm of soil [preferably from where you collect the snail] in the bottom.
Add 1-2 small logs so the snails will have a place to crawl and to hide.
Bury a small shallow dish in the soil – level with the soil surface- for water. They will drink the water but should NOT be able to fall in and drown. A tiny metallic pie dish or pot-plant saucer works well.
For Australian native land snails, feed them regularly on mushrooms and cucumber skins. For introduced species such as the common garden snail, feed them ground up bird seed and carrot. Add a small amount of lime to their ground food to help with shell development. an even better alternative is to add a cuttlefish bone to the terrarium.
Clean their terraria regularly.
Keep them in a darker, cooler place [if it becomes too hot they will secrete slime and eventually drown in it]. Do NOT put them in the sun.