How to read massive (horribly written) code

I have the joy of rewriting a program that is about 10,000 lines. The program works, but it is horrible to maintain. There are exactly 3 functions in the entire program, one of them being 9000+ lines. There are 15-20 nested if loops and it is so hard to read it. I’m looking for any advice from more experienced engineers (this is my first time rewriting someone elses code). Any tips to take it apart and understand what it is actually doing would be appreciated. Thanks