Discworld command help
brew - Brew tea to make it stronger
brew [contents of] <object> over <heat source>
While witches can and do cast spells, most of their time is spent helping people with skin rather than magic. A large part of the more physical, "take one spoonful at night and another if you wake up in the morning" aspects of witchcraft take the form of concoctions, decoctions, infusions, tinctures, salves and teas that are administered to their patients. Anyone can make tea but only a trained witch knows the best way to brew it in order to bring out its full potential.
It costs 15 gp to brew each container of tea. More than one container may be brewed at a time. Tea may be brewed over any fire or stove. Other things that put out enough heat to brew over may be lurking about as well.
> brew bottle over stove
You put the medium sized green bottle on the wood-fired stove to brew.
The medium sized green bottle finishes brewing.