It is fiddly from Guildford to Shalford. For that you need to follow the Cycle route 22 for about 2-3 miles. This takes you (on shared paths) to the downslink itself. Signposts are inconsistent so it's worth having a map on a phone/GPS (or even a quaint paper version).
You then have about 35 miles on the Downslink to Shoreham (Cycle route 223). Next stop, the sea!
It's mostly traffic free, and the surface is initially buttery smooth tarmac. Of course that soon gives way to dirt, but even that is well-packed and smooth.
I did 25 miles of it last month, and despite the wet weather it was seemingly well drained and largely dry, just a couple of muddy quagmires to negotiate.
The only bit I've not done is approx 7 miles from Partridge Green to Steyning, but
@T4tomo states that's also smooth.
But there are a couple of rough sections, so a bike with big tyres would be good for comfort.
The rough bits that stick in my mind are:
1) A steep descent near Rudgwick which is twisty, narrow, lined with trees, populated by dogs and pedestrians, and has a surprisingly loose stony surface.
2) Some gravel between Steyning and Shoreham
That said, I had no problems doing any of the above on my road bike (25mm slicks), but it's an old bike that I don't have to feel precious about.
