From the Demoboost app:
Step1: From the Demos page, click on Create new in the top-right corner and click on Create a demo.

Step 2: Name your demo and click on Create.

Step 3: Your demo is now created.

From the extension:
Step 1: Open up the extension.

Step 2: Click on Create a new demo.

Step 3: Name your new demo and click on Save.

Step 4: Your demo is now created and you're ready to capture screens.
