Sub-recipes can be created in two ways. One way is to create it the same way as a normal recipe. To see how to create a recipe, please refer to this article. Once the sub-recipe is created, add it to the parent recipe as you would any other component.
You can also create a new sub-recipe by adding it as a component in a recipe. You can search for recipes, and if it doesn't already exist you can create a new one.
NOTE: Sub-recipes included as components appear italicized, while ingredients appear normally.