To open a template, just click on one by selecting it from one of the Categories under the New Document view of the Dashboard.
Start at the high level categories and drill down to find a specific template.
Click on a template or example to start.
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 just a generic "Document" with a number indicating the number of documents 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.