No instructions

When someone asks you to look after their pet, you get a set of instructions. On there it'll say: Rover needs to be fed twice daily with X amount of food, needs to be walked every second day, his water bowl needs to be full at all time, he needs to be let out to pee in the evenings etc etc. Even if you're left with fish there will be instructions. A pinch of this fish food and a pinch of that. Plus check the temperature every day. Even if you're looking after somebody's house you will get some sort of instruction. How to use the fancy TV or when to put out the trash.

But when somebody gives you a plant to look after, you never get instructions. It's like assumed that people know how to look after a plant. I don't. I know the general principle - water the thing. But I don't know if it needs a lot of water, if it needs sun...? Now I'm stuck with someone's plant and I'm slowly watching it go yellow. Am I giving it too much water? Should I put it in the sun? Or was it a mistake to put in the sun for short periods?

HELP. I'm killing somebody's plant...