The stones need to be washed. When completely dried, apply a layer of Gesso on the full surface of the stone. 2 layers are necessary to properly mask the roughness of the stone.

Apply Gesso as a primer on the stones once or twice depending on the stone's surface and let dry. The application of Gesso allows better adhesion of the paint to the stone and avoids any cracking and flaking to appear with time.

Once the layers of Gesso are completely dried, mix blue and white to obtain a sky-blue shade which is applied to the surface of the stone. This background will be for the water.  

Outline the shape of the fish using a thin hint brush and white paint to create the shape of the fish. Images of koi fish can easily be found online; in case you want to use a model.

Add colourful stains with red paint all over the body of the fish using a fine brush for details. Then go for yellow and add stains of a different colour. Koi fishes are most of the time colourful.  At last, position the eyes with the black paint.

To create the shadows and give volume to the fish, mix the colour blue with a bit of black. Apply the new colour to the contours of the fish to give relief to the animal. Feel free to add some bubbles to the blue background in order to make the picture livelier.

Small little dots can also be added to the back of the fish to give the illusion of small scales shining in the sunlight. Here we go, your Koi Fish is now finished! It gives the impression the fish is floating on the stone. This effect is even more emphasised when watched from far.

Once done with your creation, apply 2 layers of acrylic varnish to protect your stone and make it last.