When I do it, I guesstimate the amount: about 2 tablespoons, stirred into a cup of water until it dissolves. Then add that to however much additional water makes a full pot.
I've also simply added the powder to the reservoir after filling the reservoir with water. When the water heats up, the powder dissolves.
Cleans the reservoir, the pipes, the brew basket and the pot too. There's no danger of corrosion. You can damage some coffee makers with vinegar but you won't do any damage with citric acid. Plus, it does a MUCH better job than vinegar does. People with super-sensitive taste buds MIGHT be able to detect the flavor of citric acid in the first pot of coffee made after cleaning, but I wouldn't know because I taste no hint of it.