Define a set of characters who will use the product or service you are designing. Consider the details of their lives - their job, their regular activities and their attitudes.
Identify key moments where these users interact with your product or service, then realise them as scenes in a short text or a storyboard.
Test the scenario on users or yourself. Use what you learn to improve the design further.
To investigate the full scope of user interactions, you may need to construct three or four scenarios around the needs of a different character and improve them with each iteration.
You could...
...act out your scenario by role playing
Resources
Someone who can draw, sheets of paper, markers, a video camera or computer depending the realisation method you have chosen