When a Scrum team starts its sprint, it needs to have user stories with the right size so that they can be completely developed at the end of the iteration. The art of slicing larger stories and epics that exists in the backlog to right-sized item for a sprint is not easy.
Richard Lawrence has done some work on this topic and has outlined 9 patterns for splitting user stories. He has produced a cheat sheet with example stories for each pattern. These patterns have also been summarized on a poster that should help how to split user stories. This poster is also available in French, German, Spanish and Russian.
Find the all these resources on http://www.agileforall.com/splitting-user-stories/