I agree documentation is super important! It's I recently started at a new company and have again been implementing the framework described in Manual Work is a Bug[1], summarized as:
1) Document the Steps
2) Create Automation Equivalents
3) Create Automation
4) Self-Service and Autonomous Systems
When I started going through stuff with my coworker, I made a Word document with bullets and sub-bullets for all the steps in a process. When they unexpectedly needed to take a week leave of absence a month into the job, I was able to complete almost every routine part of the job. It took a ton of stress off of them!
Across all my professional jobs, I find #1 to be the hardest. My process is to write down the high-level steps, then strictly follow the documentation next time I do the process, adding more and more detail every time.
Once it's decently documented, I record how long each step takes, and add comments like "This needs to be better documented" or "This is the most tedious step, and would be pretty simple to automate". Timing each step lets me start and complete something between meetings, filling in gaps.
Overall I've found the system to be really helpful. It makes transitioning job duties to new employees trivial and has saved me literally hundreds of hours over just a few years in the professional workforce.
[1] https://queue.acm.org/detail.cfm?id=3197520