I don't want to spend too much time describing each one in great detail because the trails are ever changing! Recently did the MBR from Glyn-yr-Aur to the new bike centre and wondered why I was having so much difficulty with the last section. What had happened was the direction of the MBR had changed and I had been trying to ride (with partial success) up a newly contructed down staircase. This is now the start of the MBR and even going down in wet weather is slippy and treacherous. Official maps of the trails are available in Glyn-yr-Aur and the New Visitor Centre and parts of them are reproduced below. (Aled..ga i eich caniatad i cyhoeddi?)
Easy Scenic Route to the New Coed-y-Brenin Centre
The Beast
(Any references to "You are here" are only valid if you are at the New Bike Centre!)
For some of the routes look out for Glyn-yr-Aur marked with a red arrow.
Heres what the Guardian says about biking in the Coed-y-Brenin

Mawddach Cycle Trail and other routes
The Mawddach Cycle Trail is a beautiful flat scenic trail along the estuary from Barmouth to Dolgellau or vice versa and is suitable for adults and children and all abilities.

The Glyn-yr-Aur Bikewash!

Actually just an outside tap with hose and reasonably high pressure water but effective none the less.