When looking for a subject to paint, the standard bowl of fruit is always a good option. However, it might be best to choose a fruit that is very simple and lacks embellishments like leaves or stems. To paint watercolour oranges, you should possess basic skills with watercolour paints and have some familiarity with light and shadow before you begin.
Lightly draw out the shapes of the oranges. Keep this simple and flat by not adding in details like shine or blemishes. This will be painted in later.
Make a light orange wash with a little paint and a lot of water. Paint in the oranges with the wash, filling in the entire shape. The colour wash will act as the base colour you will build upon. Allow the paint to completely dry.
Lightly erase some pencil markings leaving behind the colour wash and a very light pencil outline. It's hard to remove the pencil without removing some of the wash so be careful. It's better to leave behind the pencil than remove the wash.
Mix a base orange colour for your oranges. This will be the body colour of the fruit. Paint in the oranges completely.
Dip a clean brush in water and use the brush to lift up the paint from the painting to make your highlight areas. Make sure the highlights are all on the same general area of the oranges. Think about how a light shines on objects.
Mix a light orange with your paint and water. This will be your highlight paint. Paint in the highlight areas you lifted up in step 5. Use a wetter brush to help blend the edges of the highlights and base colour.
Mix water, orange paint, yellow and reds to build a richer darker orange. Use this colour to add body and depth to the oranges. For round objects like oranges, it should be richer or darker the farther the curve is away from the eye, and lighter the closer. Use a lightly wet brush to blend the colours and build different tones and shades.
Mix the shadows with water, orange paint, and a blue or purple paint. Apply the shadows to the edges of the oranges. Think about how shadows form when light is shined on an object. Use a wet brush to help fade the shadows into the base colours. Keep in mind as shadows move closer to highlights they lighten up.
Allow the paint to dry overnight. With the paint dried, you can look over your work and determine if you need to adjust tones. If you're happy, sign the art and date it. Repeat these steps over and over until you have a feel for the paint and develop your own techniques.