How to make a "Thirst/Hunger" script from scratch?

Well basically all you'd need is a value inside their character or something of that sort that scales from 0 to 100 and there is another script that constantly is reducing the value at certain intervals and then make tools that when you click you eat/drink them and it adds to the value.

Continuing from Zack785

So you'll also need a script to detect if the water value is a certain value to then start harming the player. Now also you are gonna need a lot of value knowledge and math to make something like this , work. So have fun!