We first did this walk on 20/04/08 and Paul wrote it up on his blog.
Parking at the long stay car park at the bus station in Royston is £1.50 all day and free on Sundays. If you do it anti-clockwise as we did then it uses the Icknield Way path on the way out and the Hertfordshire Way on the way back which makes for generally easy walking.
The only vaguely tricky part to navigate was the bit across the field starting at OS 340375 as you head into Therfield but there's a diversion via the bridleway if you're unsure and the bit across what seems to be common land as you leave Therfield. There's an Icknield Way sign at the road but then you're left to your own devices to find the bit marked as "Chain Walk" on the OS map.
Here's the route on Google Maps and here's the GPX file (GPS Exchange Format, 3KB) for those with GPSs.