A tutorial application which showcases the basic usage of a Flow & Spring Boot Application is available at https://github.com/vaadin/flow-spring-tutorial. You can use this application example to test the different concepts and features presented in the documentation.
For starting a new project with Spring and Flow, you can clone the Project Base for Flow and Spring repository. It is a project template with the necessary configuration and dependencies included for starting building you own application. This starter will be soon available for download from https://vaadin.com/start, where you can customize the naming (package, project) for it.