Things I did:

Keyword research

Competitor analysis

Experimented with various Seo tools: Yoast Seo, AIO Seo, Moz

Set up Google Search Console to fix problems such as non-indexed pages

Set up Google Analytics to track performances

Fixed the spam score with Google Disavow Link Tool

Rewritten all the website (texts, titles, metadescriptions...) with SEO keywords

Fixed other SEO issues (images alt text, internal and external links, pages speed)