SEO is a process that can be started from anywhere KW research & Optimisation, Content Planning & Optimisation or linK building.
Here's my 3 Point Actions to start
1. Take a Look at your Site Data (Search Console, Google Analytics or any other tool) for the site you are working on and try make sense as to where you can identify problem or improvement areas.
2. Try to learn more about solutions for that particular problem area, to me this is the easiest and shortest way to get engaged in any project and learn while you work on. Simply go on implementing what you learnt and gauge the effect and then going over to the next improvement area and so on.
3. Make Notes of things that worked and that didn't, for future reference as well as record keeping to measure and judge the what made them work.
If you go on identifying where to Start learning from, believe me, you'll be spiralling around infinitely and may not come to any conclusion and more importantly never have the confidence to work independently.
All the talk about Action Taking is just about initiating and you may initiate from anywhere as I just mentioned at the start.