Fermenting

The first step in every brew is fermenting your ingredients. The tool you do it in is a cauldron of water over a heat source — think of the cauldron as your cooking pot, and fermenting as what happens inside it.
Setting up the cauldron
Place a cauldron and fill it with water.
Put a heat source directly underneath — a fire, campfire, lava, or magma block. The water starts to boil.
Fermenting
Right-click the cauldron with each ingredient to drop it in, in the amounts the recipe calls for.
Let it boil for the recipe's time, measured in real minutes. Coloured particles rise while it works.
Check progress any time by right-clicking the cauldron with a clock — it tells you "This cauldron has been boiling for X minutes."
The longer it boils, the more it ferments — and the finished bottle's description will literally say how many minutes it fermented for.
Bottling
When the time is right, right-click the cauldron with empty glass bottles to fill them with the base brew.
Each fermentation makes a batch of 3 bottles — a full cauldron fills three and then empties, regardless of how many ingredients you added. The recipe's ingredient amounts are for the whole batch of three, not per bottle, so there is no need to multiply anything.
Timing matters: bottle too early or leave it boiling too long and the quality drops. Simple drinks like beer just need aging after this — spirits still need distilling.