When we refer to ‘tonal values’ in drawing, it is the same as the common term of ‘shading’. Shading is the part that makes a drawing go from a flat contour drawing, and gives it depth, so it appears as a 3-dimensional illusion. 

Look at objects in your home, like lamps, bottles, containers, etc – everything is affected by the light and shadow that fall on them. When light strikes any three-dimensional object, it creates what is referred to as the Four Elements of Light:

• The light side, caused by light from lamps, ceiling lights or natural light from windows
• The shadow side is the side that faces away from the light source
• The cast-shadow is shadow that is created on a table top, or caused by objects that are placed close together 
• The reflected light happens when light ‘bounces’ off one light colored surface, onto the shadow side of another object

There are many options for shading, and each option will change the style of your drawing. Here are some of the basic ones:

1. Cross hatching: With cross hatching you are drawing parallel lines, varying the direction of the lines and layering them to obtain different intensity of shading. Start by adding the lightest area and then add layers of cross hatching until you reach the darkest area of the drawing.
2. Parallel Line Hatching: Hatching with parallel lines is the same as cross hatching, except you are making all the lines go in the same direction. It is a bit more time consuming than cross hatching but can lead to interesting tonal results.
3. Contour Lines: Contour lines are usually seen as subjects that are drawn without shading. They can be done in different ways, but basically the idea is to have the lines follow the shape of what you are drawing.
4. Scribbles or ‘Scumbling’: Scribbling is a way to shade a drawing with scribbles that are drawn quickly. While they may seem unorganized and messy, scribble drawings as shading works particularly well for different styles of drawings. They also add an original texture to the drawing.
5. Blending or Smudging: Blending or ‘Smudging’ is the technique that is most widely used by artists – one can use a paper blending tool, a cotton tip used for makeup, or using one’s fingers to blend & smooth to create the smooth look of their drawing. 
6. Stippling: Stippling is creating the tones by making many small dots – it is interesting technique, but also time consuming. The artist can use different sizes of pens & markers to make different sizes of dots and ‘dabs’ of ink when they are using the stippling technique.
7. Creating tonal values as patterns & textures: Choosing a selection of design patterns as the effect, rather than drawing the more abstract methods of lines & tones as mentioned above. 

The choice of technique of creating shading tones is left up to the artist’s whim, depending on the look that they desire in their drawing. Many artists try different styles, and they will even mix several shading techniques in one drawing for unusual visual effects.