Citadel Gray Seer Base paint is designed to give a smooth, solid light gray color over an undercoat and for that reason this will almost always be the first paint you use on a miniature. Shake the light gray paint before using (you can make this easier by adding an agitator ball).
The light gray paint can be a bit thick sometimes and then you just add a few drops of water to your pallet. You really don't need a lot of water. Something to watch out for is to make sure you don't apply too much paint so that you don't start hiding details.
Another important functionality of this light gray paint is that it not only covers the underlayer, but also covers other colors. So if you later miss something about this color, you can easily paint over it with this Gray Seer base paint.