The biggest challenge for me was understanding how correlation is different from causation in terms of SEO. Multiple aspects are changed at one time, the ranking changes, and one assumes that the last change must be the one which made the difference. taking proper notes, making smaller changes, and studying the effects taught me much more compare to any SEO tutorial.Everyone learns SEO differently, and some concepts take more time to fully understand What was one SEO skill or concept that took you the longest to master? I think practical testing and experience often teach things that guides cannot