As soon as you have created a Routelet, you will find it in your usual Palette of components and thus you will be able to include it within your Job.
Like any other component, click the relevant Routelet name in the Palette and drop it to the design workspace to include it in your Route. Note that the Routelet can not be connected to other components.
Define all other components properties and context variables, if required, before running the Route.
Save and run your Route as usual.