The views to the south from the summit of Mount Islip are nothing short of stunning. You can see Crystal Lake, the front range of the San Gabriels, as well as the San Gabriel river canyon. The hike starts out heading up a forest service road for a half mile, then you head left at the signed trailhead to Little Jimmy Camp, one mile ahead. The less ambitious in your group can relax in the camp (it has rest rooms and a spring a 1/4 mile away), while the balance of your group heads up to the summit.
The summit trail is at the back of Little Jimmy camp (walk past both rest rooms). The summit trail takes you up some switch backs to a ridgeline. There is a junction with another trail at the ridgeline that comes up from Crystal Lake, but you should stay on the ridgeline trail up to the summit.