To create a child style of an existing style, click on the style options menu for the style of which the new style will be a child, and pick Add Child Style from the popup menu.This will take you to the Add New Style interface, with the correct parent style already selected in the form.
You may continue to add further child styles to child styles in order to satisfy any inheritance critera you may require.The image above illustrates the style layout as described in the reference guide to Style Inheritance.