Havana — How to conserve cellphone battery when recharging options are scarce; how to preserve food when the fridge isn’t working; how to cope with the heat when air-conditioners aren’t an option; how to generate power when the whole country has almost none left. These are the sort of tutorials that have made the Instagram feed of influencer Dayán Marrero indispensable to many Cubans as they try to adapt to the rolling electricity blackouts and fuel shortages that are increasingly part of everyday life on this Caribbean island. As the US-imposed oil blockade that is worsening the situation bites ever...