the north side of the lake has lots of cobblestones which is sometimes tricky. there are chutes here if you're into that. at one point the trail splits on a hillside where you can take the bottom (flowing and twisty), middle (off camber, narrow trail, twisty), or top which i haven't done yet, but it has a fun downhill at the end. the south side of the lake is easy, winding in and around the paved bike path. smoother trails there, but twisty.
I usually park in old folsom in the parking lot next to the old train turnstile. ride northeast down to the river and cross the bike bridge, turn left and go under the bridge and right after that the singletrack will start off to the right. from there on just follow the bike tracks wherever you think you're lost. there are a few different ways to go so just try them all out. you'll get to where you have no choice but to go back on the paved path, go around and cross hazel bridge, stay right on the bike path and you'll end up in the park parking lot, go through it and on the other side there is more singletrack. on this side of the lake you'll have to look a little harder for the singletrack, but it's there.
http://www.singletracks.com/item.php?c=1&i=1501