It is harder to thicken paint than to thin it, so add water judiciously with the eye dropper. Mix the paint thoroughly and test it for the shade you want. Let it dry. Gouache looks darker when wet. If your brush sheds pick out the bristles right away before the ground dries.
Some people brush or sponge water on their paper and then wipe it off, leaving a damp surface.
This prevents the ground from drying before you finish. This is optional. Then, before it dries, repaint the same surface in a vertical direction and allow to dry. It will appear uneven until it is completely dry. Paint two colored grounds.