Breaking up solutions into small steps is a useful skill that takes time to learn (and might also take some knowledge about the problem domain).
So don't worry, you'll likely get better at it with practise.
What would probably be wise to do is ask the senior developer to break it down some more. Or to break down one of the steps that (s)he described. Breaking it down together would be even better if there's time.
Take notes, or break up the steps in the issue tracking system (e.g. stories in Jira).