Bill Wake has given us the mnemonic INVEST which HELP us in writing a well-formed User Story.

Splitting a story is not an easy task.

Let us first discuss different techniques on how to split a user story.

• Split by user roles-Administrators interact with the system in a different way than users. It a very good way of splitting the user stories by user roles.
• Split by capabilities offered.-Capabilities example like sorting and searching. Further sorting and searching may further split into different user stories.
• Split by user personas -Even in the same role, users interact with the system in various different ways. A handicapped person interacts in a different way, a casual user in a different way as he needs intuitive things, a POWER user needs lots of short cuts, etc.
• Split by target device- User can interact with our system not only using a standard computer but also using mobile, ipad etc.So this is also a good way of splitting a user story.

For DETAILED INFORMATION check my blog below.

How to Write A Well-Formed User Story