Depending on the desired effect and the situation (nearby, far off), you can apply stronger or less strong lines. To create a more subtle effect, you can mix the dark brown with the basic colour on the spatula or on your hand. This makes the colour that you use for shadowing less powerful.
The more subtle you work, the more natural the result.
Important areas for shading are the eye sockets, the temples, the side of the nose and below the cheekbones. The vertical frown wrinkle and the naso-labial ('nose-lip') fold are also important areas to shadow.
On an older person the skin, where there is little or no bone, is sunken. On a younger person these parts are darkened (shadowed), the skin will appear to be really sunken at that spot.
When you age somebody with a spherical, round face the bone structure is hardly visible. In this case, do not shade the cheekbone on the round cheeks below, but emphasize on the cheeks. Draw a line below the nose-lips line that goes underneath the cheeks, for a baggy effect.
To determine which side should be sharp and which side faded, you start from the centre of the face (nose). Create a fading effect towards the outside.