The best way of doing this, based on years of science, would be, well, to eat nothing for 24+ hours at a time, but make sure you stay hydrated; as in, actually fast. The second best would be to seriously curb your carbs, under 30g a day; also tied for second best is to eat just once a day, none of this unscientific three square meals hogwash.

Three of these together could halt the progress of some cancers, and before the shitstorm that was 2020, scientists were publishing papers involving animal models on this.

Steve Jobs did none of these, and was, sadly, off in la-la land when someone with his money and connections could have had access to next generation scientifically-based treatments. Fruitarianism is, frankly, dangerous.

So water and coffee or tea (without milk, of course) would be fine. You're trying to avoid an insulin response and firing up the entire machinery of digestion.

I think it's non-controversial to say that this starves the cancer cells.

Perhaps less certainly we can also say that digestion demands a lot of resources and is an interrupt for a lot of other processes. When your body has nothing to do for 24-36 hours eventually lower priority tasks get attended to ... like garbage collection.