Quickest way to intermediate-level SEO? (existing marketing knowledge)

Been in marketing for the past decade+ but have spent little time on SEO. As in, I can talk about it at a high level but can't tactically do much.

What's the most efficient way to build this skill to an intermediate level? I have some smaller clients I'd like to offer this service to soon.

Note: I have extensive paid search experience, if that helps.