Beowulf is a classic tale which has been translated many times and has had many film adaptations. Seamus Heaney's new translation and Frederick Rebsame's translation are good starts if you have not read it yet. Of course, it was one of the inspirations of Tolkien's Legendarium.

What is a step-by-step process to adapt any story, for example Beowulf, into a game scenario?

It does not necessary need to be the same story, you can (read: should feel free) to get the main themes from Beowulf into a story. I am looking for methods of how to adapt literary work to role playing adventures using Beowulf as an example.