Distance: 8 miles (13 km) round trip
Trailhead Elevation: 9,700 feet (2,950 meters)
Spillway Lake Elevation: 10,480 feet (3,195 meters)
Elevation Gain: 800 feet (240 meters)
Your route to Spillway Lake follows the Mono Pass trail, shown in red on the map, for the first 2.3 miles (3.7 km). Veer right at the signed Spillway Lake fork, marked in green above. You'll reach the lake in another 1.7 miles (2.7 km), assuming standard good fortune - staying on the trail, no alien abductions or attacks by enraged marmots, etc.
The trail follows a pair of creek drainages - first Parker Creek, and then the Spillway Lake outlet - and ascends gradually throughout, alternating between woods and meadows along the way. The rich greenery is a dramatic contrast to the stark, dry rocks of the eastern Sierra slope a few miles away, which you'd get to see if you took the Mono Pass fork instead.
The blue line marks the Parker Pass trail, frequented by backpackers. Though it's only shown to the park's boundary, it extends for miles beyond.