Materials needed: fine weight yarn in desired colors
Not a lot of yarn is really needed. These are going to be about 4cm (.4mm) in diameter and .35mm (3.5 cm) tall to fit a full grown Helix Aspersa snail (commonly known as a garden snail).
These sweaters are for temporary use, like when sharing pictures on how cute they look on social media. It is not wise to keep the sweaters on longer than needed.