Start with formulating the user journey from a user persona's perspective. Mike, a 26-year old barista, is looking at a funny cat compilation article. One cat is extremely fluffy, and Mike presses the Like button under the photo. What happens next, provided that Mike is a registered user? Write it down.

When writing the user journey, use non-technical language and don't make it too long. Its purpose is to concisely explain the solution offered to the users' problem.