From Onion Valley, take the Kearsarge Pass trail to the Kearsarge Lakes/Bullfrog Lake trail junction, located a few hundred yards beyond the pass. Follow the Kearsarge Lakes trail for a short way to an unsigned fork in the trail (not shown on any maps). Continuing straight ahead on the more prominent left fork leads you astray to Kearsarge Lakes; the right fork heads to Bullfrog Lake, and is the correct trail. (If you miss this fork, take heart that it is easy to head cross-country from Kearsarge Lakes to Bullfrog Lake, it just makes your day a bit longer). Follow the trail past Bullfrog Lake to the John Muir Trail (JMT), which you follow south for a little under two miles to Vidette Meadow.
A short way past the JMT's wide, shallow crossing of the stream that drains Bullfrog Lake, cross Bubbs Creek on a large log. If you head southeast from this point for a short way, paralleling Bubbs Creek, you should encounter the old remains of one of Shorty Lovelace's log cabins. (Lovelace was a fur trapper who worked in this area before Kings Canyon National Park was created in 1940). A few yards west of this cabin, a use trail may be found that heads south through the Vidette Creek drainage. Some caveats: The trail probably requires a small miracle to find from Bubbs Creek, and is difficult to follow higher up in the canyon, with only a few scattered ducks suggesting the route. Fortunately, cross-country travel is easy throughout the whole drainage, and the trail is really no more than a luxury. Keep to the west side of Vidette Creek for easiest passage.
Hike south past the highest and largest of the Vidette Lakes; this is the last reliable water. A short way beyond the lake, you should be able to angle right up a gully and easy slabs to reach the talus ramp leading to the prominent NE Buttress on the lower NW peak. (This avoids the talus slope at the head of the valley). Proceed over endless talus to the base of the buttress.
A use trail threads its way up the righthand side of the buttress through an obvious weakness in the cliffs. The climbing here is loose and tedious over unpleasant scree ledges, but once atop the crest of the buttress it improves nicely with a mixture of slabs, ledges, and large boulders. Work your way up the buttress to a point along the summit ridge about thirty feet above the prominent notch that separates Deerhorn's twin peaks. If you're on route, you should find it surprisingly easy the whole way, mostly class 2-3 with little exposure. If you're not, you'll find yourself on steep rock with some exciting exposure. If you encounter any difficulties, just spend a minute or two looking around; they should be easily avoided--unless you're looking for more excitement. Ducks may help in the route-finding. In general, the right (NW) side of the buttress offers the easiest passage, especially near the top; the left side is steeper, and eventually becomes sustained and exposed class 4 up to the saddle.
From the summit ridge, traverse left over easy sandy ledges down to the saddle. (Attempting to traverse too early will lead to cliffs). The route to the summit from this point basically forms an "S"-shape: A small bump by the notch is passed on its right (southwest) side. After passing this, angle up and left to a small saddle along the ridge as soon as possible (staying too low on the SW side leads to steep slabs), and follow easy sandy ledges around to the north side of the summit. Head right up a short class 3 section to the tiny summit. (Secor claims there's room for four climbers up there, but I think they'd have to be very friendly to pull that off).
The buttress may be easier as a descent route: There is no temptation to keep left for a view of the notch, thus avoiding the steeper rock, and the easy options are more obvious when looking down from above.