Overview:
As you approach Donner Pass from the west on I-80, you can't miss Castle Peak on the left.
To get to the trailhead, take the Boreal exit (between Soda Springs and Donner Lake.) From the Boreal parking lot go under the freeway and follow the road up to the right and find a place to park. If you have a 4WD with high clearance, you could follow the dirt road and shorten the hike by a mile or two.
Start the hike by following the dirt road. In a mile and half the trail will start to climb and you'll reach a junction at Castle Pass. The PCT crosses this junction. Turn right and follow the ridgeline up to Castle Peak. From the ridgeline you look to your left (north) and see a beautiful green valley and ahead and to the right you can see your destination. Simply follow the well defined trail to the top. The climb gets quite steep (1300' gain, mostly in the last 2 miles) and I recommend hiking poles if you have them.
If you have the time, you can also hike over to Andesite Peak or Basin Peak.
Tips:
Bring sunscreen and be prepared for any weather. I've also once encountered mosquitoes on the hike in the lower section.
I'd also recommend hiking poles for the steep rocky terrain.
This is the junction of 4 trails. The PCT which goes up and over, a trail to the left which follows the ridge up to the Andesite Peak, and the trail to the right which follows the ridge up to Castle Peak.