Double Diamond is an iterative design process made up of four stages; Discover, Define, Develop, and Deliver.
The Double Diamond is made up of two diamonds; the first diamond represeting 'The Problem', and the second representing 'The Solution'.
Throughout this process you will adopt different thinking styles, sometimes thinking divergently (thinking wide), and sometimes thinking convergently (thinking narrow).
This first divergent phase is about understanding the problem as well as possible. This is made up of research and data gathering in the form of User Interviews, Usability Testing and data analysis.
Aim to get a nice combination of quantitative data (in the form of numbers) to help understanding user behaviour and understand WHAT users are doing, as well as qualitative data (in the form of words) to be able to understand WHY users are doing what they are doing.
The learnings from the research during the discovery stage will help you to define what you need to be focusing on. Use your new understanding of the problem space to converge on the most important part. This could be the biggest pain point for your users, or just an area of most opportunity.
The second diamond is about designing and creating solutions. You’ll start by thinking wide again by collaboratively creating ideas and prototypes that you think will solve your defined problem. These will then used for testing. Get your ideas in front of users and test if it solves the problem you are trying to solve.
Finally, deliver the solution by getting it built into the real product so it can start to impact real users. Continue to monitor any changes you make to make sure you are making the impact that you expected.