Gain the summit of Volunteer Peak
(10,481'). This can be easily reached from the Pacific Crest Trail in the vicinity of Smedberg Lake via a grassy chute that leads to the saddle just east of the peak. The summit is an easy boulder hop from the saddle.
Descend from Volunteer Peak over boulders to the aforementioned saddle (10,160ft+). Make a very gradually descending traverse south past Peak 10,640ft+ along grassy benches and talus, eventually dropping down to about the 10,000ft level. (It's not necessary to explicitly aim for this elevation--avoiding some cliffbands and taking the easiest passage along the way will naturally tend to bring you down to this level). A very steep gully drops down from the unnamed lake 0.5 mi north of Pettit Peak (Lake 10,080ft+) to the bowl containing Rodgers Lake, forming a prominent cleft in the cliffs east of the latter lake; the traverse from Volunteer contours east to the unnamed lake along the north side of this gully. (It's much easier than it looks from a distance).
From Lake 10,080+, a talus chute leads southeast up to the ridge connecting Volunteer and Pettit. Once atop the ridge, the summit is a short walk to the south, and is the third or fourth bump encountered along the ridge.
It is also possible to follow the ridge from Volunteer to Pettit, although this involves more elevation gain and loss and may not be any faster. Following the ridge would be mostly class 2, with a short class 4 step reportedly encountered during the traverse over Peak 10,640+.