“Scrawl” is a great word to describe how I used to annotate papers during my PhD. I hated reading papers. I much preferred doing maths: balancing equations, defining functions, exploring little curiosities that popped into my head. It never occurred to me until after my PhD that reading papers was doing maths. It always seemed overly difficult.
I would scrawl over papers with whatever was to hand. Red pen in reach? Use that to underline. Get bored. Next day, a pencil is nearest. Start making notes in the margin, switch to pen to emphasis even if it is harder to read. Get bored. Next week, a highlighter, make things stand out, and so on.
I’d look over things months later when I needed a particular result and it was a mess. “How did this happen?” I would ask myself and it was only towards the end that I realised, “Oh, it was me. I made this so hard!”
It’s a great idea to annotate your thesis. You need a clear system in place for what you’re doing. Use red pen to underline typos, but only use it for that purpose. Make pencil notes in the margins, but only put notes in the margins and only use pencil for notes. Use highlighter to draw attention to only the things you really need to stand out, and so on.
Or come up with your own system, but learn from my mistakes, please!