To open a template, just click on it in the Template Browser screen.
When you select a template, you are actually creating a copy of the template to make your own changes to. If you don't explicitly give your copy a name it will be named after the template with a number indicating the number of copies you've created. The original template will be unchanged and you can always go back and start your diagram over with the same template.
Selecting the right template to open is important because it gives you specific tools in the form of a SmartPanel and relevant libraries already docked.
Learn how to create your own custom templates here.