Starting at Steyning, I would suggest exit via the A2037 and then right along the Edburton road (this is a really nice ride along the Downs and you can see as far as the North Downs at some spots) toward Fulking, carry on towards Poynings and then head South towards Brighton past Devils Dyke.
You can then turn round and go back the way you came or carry on to the coast, turn right when you hit the sea, there is a cycle path along the coast here but you have to take it slowly as there will be a lot of pedestrians about, follow the coast as far as Shoreham and then pick up the Downs link back towards Steyning.
Sorry i don't have a link to a route but you should be able to get the idea via Google maps, hope this helps