This is what I usually do, you may want to take the 57 to the 10 instead of the 15.
From OC, take the 91 East. Get on the 15 North. Take the 10 West. Exit on Mountain and take a right. Stay on Mountian for as far as it goes. It gets a little confusing when you are on the outskirts of Upland. The road will curve right and then back to the left to a stop sign. Take a left to stay on Mountain. Mountain then joins with and becomes Mt. Baldy Road. This will take you through the village of Mt. Baldy. The ranger station is located here. You will need to get a parking permit for Angeles Crest Forest. Continue up Mount Baldy Road. You will see a parking area where the road becomes divided. If you get to the ski resort, turn around, you've gone to far. From the parking area follow the fire road. You will go to San Antonio Falls, take a hair pin turn, go up and curve left. Look to the left and you will see a trail going back to the left. This will get you to the top. You should run into the Ski Hut along the way. It is a green building at the bottom of the bowl.

For other information, you can call the Mt. Baldy visitor's center at (909) 982-2829.