Here is a nice comprehensive description taken from the Alltrails website:

"A walking trail through the setting of Richard Adams' famous novel, Watership Down. Begin in Newbury, and follow the book through Newtown Common, Ecchinswell, Overton and Whitchurch. You'll see Sandleford Warren, the Enborne River, Nuthanger Farm, Watership Down, Efrafa, the Railway Arch, and the grave of Richard Adams himself."