From I-5 just north of Shasta Lake, take the Packers Bay exit (only accessible from I-5 southbound). If you're coming from the south, take the O'Brien Exit and then jump back on the I-5 and take the next exit which is the Packers Bay exit. Drive down to the marina and park.
The trail is marked well and skirts around Shasta Lake. It is hard packed singletrack with a couple steep climbs and a 'hike-a-bike" section depending on your skill level. This trail has about 50/50 sun and shade on the trail. On a hot day, jump into the lake or in the creeks that feed it to cool off.
This is a short ride but if you make it an out-and-back, it is worth it.