Start from the car park at Cocking, up onto the Downs heading east, then turn off left after reaching the ridge and head north to the sandy woods, then east via the disused railway if you find it, then stop at a pub at Graffham or Duncton (they’re all half decent) before doing the climb either up from Barlavington (which is a challenge to clean) or the one past the school at East Lavington onto Graffham Down. And thence back along the SDW.
Without a map or local knowledge you’ll probably not know where you’re going on the northern section of that route (Heyshott to Graffham) but Graffham down is always quite visible and you’ll know when you hit the main road at Duncton, so you can’t go far wrong. Distance wise it’s 20-30 depending on how far north you head or how lost you get 🙂