* Reading lots of books on writing. Probably this (https://www.amazon.com/Writing-Web-Creating-Compelling-Pictu...), definitely this (https://www.amazon.com/Style-Lessons-Clarity-Grace-12th/dp/0...). Got both from local library.
* Reading lots of other books I got from library (on writing and marketing).
* Practice, coupled with observing results on google analytics.
* https://practicaltypography.com/typography-in-ten-minutes.ht... for basic design.
* Free content on https://stackingthebricks.com/ (there's a lot, although it's mostly lead-in to paid content).
The last one in particular motivated me to write based on research, which resulted in massively improved results.