Slurry / pre-hydrate — Combine 1 part tapioca starch with 5 parts water, by weight. Mix vigorously. (It's not important to have have this specific ratio. Rather, it's important to have a consistent ratio over time, so that you can gain an intuitive sense of "if I pour in this much liquid, I will get that much thickening power".)
How to use it in a dish? — Heat to 63 - 84°C (145 - 193°F) to activate the tapioca starch. [1]
Can it be made well ahead of time? — Yes. However, the tapioca starch will precipitate out after standing for some time. So store in a jar that's at most half-full, so it can be mixed by vigorous shaking. (I like these style of plastic bottles that are basically just condiment bottles)
When making it well ahead of time, does it need a preservative? — Because this is a starch slurry, you do need to preserve it some way. I prefer to just store it in a refrigerator, and discard it after ~2 weeks.