Already found one that might be helpful so I share it here, notes for a short course by Schoen at Tsinghua unversity, https://web.math.ucsb.edu/~zhou/Math_GR_2012.pdf
Also, Yau's own explanation in a lecture at Harvard https://www.youtube.com/watch?v=qCIpxab8WDc&list=PL5ahlY3eFvHYrtDc_8Y-iO2uOQjz8sslC&index=3
Thanks to Willie Wong for pointing out Dan Lee's Geometric Relativity book, which gives the most up-to-date and complete summary on PMT proof. (Still Schoen's own lecture note is very helpful for building intuition about Jang's equation.)