I went to a YC angel investing workshop in March 2018. The lectures are all on YouTube. My takeaway was to try to independently consider deals, and not worry about herd mentality. Of course, there was a lot of material, ranging from YC partners, external angels, and VCs. There was conflicting information and advice about the decisionmaking process -- which is good, I think: the process is different for everyone.
Lectures: https://www.youtube.com/watch?v=7aiJlRS2i_w&list=PLQ-uHSnFig...
My evaluation template for the recent YC batch: https://alanglennon.com/2019/03/15/yc-w19-template/