Fiddleneck Trail can be taken by choosing the left trail out of the parking lot. You'll hit a bit of a hill to start it off. There's a nice overview of Rohnert Park from the top. It's all downhill and flat from there.
Just like Creek Trail, there are lots of trails that stem from Fiddleneck, and the signage isn't the easiest to follow. You can always use the MTB Project mobile app
to find your way. Be sure to stay on the trails that are designated for mountain bike use as there are some trails at Cane Creek that are for pedestrian use only.