I've been compiling some notes on exactly this topic for the past few weeks, your question prompted me to post them here: http://dubhrosa.blogspot.com/2016/07/how-to-dive-into-largec...

TL;DR: don't "dive" in, figure out the inputs and outputs and work from the outside in; make lots of notes as you go; don't browse the code aimlessly, write down a question you want to answer and focus on that, then write down the answer when you think you've got it; don't get too hung up on the editor/IDE; possibly controversial/YMMV: turn off syntax highlighting; page through a file from beginning to end spending 5 seconds per screen when you open it for the first time.