How do you make sea water for a snail?

You don't need to make seawater for a snail. Snails are not marine animals, they are terrestrial or freshwater.

If you're keeping a land snail, you should provide them with a damp environment and a shallow dish of fresh water to drink from. Never use saltwater for a land snail.

If you're keeping a freshwater snail, they need a tank with fresh water, usually dechlorinated tap water or spring water.

It's important to do research on the specific species of snail you're keeping to ensure you provide the correct environment and care.