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. The upside to this is that Crane Creek is such a small park that it can actually be fun to try to get lost in it, although one way or another you'll always end up back on some familiar trail. You can always use the Trail Run Project mobile app to find your way.