Park in the Cougar Crest trailhead parking lot and go up from there. The trail ends at the junction of the PCT, where bikes are not allowed to ride.
Most expert skilled riders and maybe a few novice riders will be attracted to Cougar Crest trail because of the technical nature of the embedded rocks on the trail. The bottom third is fast (going down) and smooth, then the top two thirds is where it gets fun.
Expect to run into hikers on the weekends, and maybe a few during the weekday as this trail is a popular hiking trail for the locals and visitors. You can already gauge how many people are on the trail by the cars in the parking lot.
Ride this one up and down - both directions are equally fun. It's a favorite trail in the area.