True casing is actually a two step process.

The hide should be soaked until the small bubbles are seen bleeding out of the skin, THEN the hide should be stored for ~12 hours in a 'case'. Originally this was a steel lined box, you can use a mostly (but not entirely) sealed tupperware container. This allows the water to fully penetrate the hide (to achieve the best plasticity and evenness) but for any excess to start to evaporate.

If you're not doing incredibly fine tooling you can probably get by giving the leather a decent soak with a VERY wet sponge, letting it set for a while to penetrate and even out, then tooling.

HERE'S a pdf from a very well-known tooler with some guidelines.

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