For maps, go to http://www.fairportconvention.com/ and follow the signpost to "The Festival", then "Festival Information" and finally "Festival Maps".
If you're moored to the South, go up to the canal bridge connecting the Fields 7 to Field 1 (Cropredy Mill Bridge 154). Then leave the towpath (there's an open door in the wall just North of the bridge) and follow everyone across the bottom of field 1. The bottom corner of Field 2 is then the entrance to the main arena field, but first you'll have to go a bit further along to a marquee where you'll exchange your paper ticket for a wristband.
If you're approaching by road, on your bike, perhaps, then come along the Williamscot road to the junction between Fields 9 and 11 (you won't miss it, everyone else will be going there!). I've usually seen half-a-dozen bikes chained to the fences out of the way at the top of field 9. (The towpath South of Cropredy Mill Bridge is not really suitable for cycling).