The most common way to kill one

Nearly every dead cactus I have been handed died of attention. The roots want to be properly soaked and then left completely alone until the soil is dry all the way through — which, indoors, in winter, can be well over a month.

A thirsty cactus tells you clearly: it goes slightly soft, it wrinkles, the ribs draw in. All of that reverses within a day of a real drink. A drowned one gives you nothing until the base has already gone, and by then there is no reversing it.

So the rule is boring and it works. Water it properly. Then forget about it for longer than feels reasonable.