On-page SEO has not changed much in principle, but the things that quietly break a page have. Work through these in order.
Start with the basics: one unique title tag per page at roughly 60 characters or fewer, a meta description under 155, and exactly one H1 that names what the page is about. If the title and the H1 describe different things, search engines have no single subject to rank.
Then check the canonical. Every indexable page should point at itself. A page that inherits a site-wide canonical from a layout or template is telling Google it is a duplicate of something else, which is an instruction not to index it — and it is invisible unless you look at the served HTML rather than the source.
Keep your sitemap honest. A lastmod date that updates on every deploy, whether or not the page changed, teaches search engines to ignore the file entirely. Only move the date when the content actually moves.
Finally, add structured data that matches what is visibly on the page: BreadcrumbList for navigation, FAQPage where you genuinely answer questions, Article for posts. Schema describing content a visitor cannot see is worse than no schema at all.
Farvixo's SEO Analyzer audits any URL against these checks, the Meta Tag Generator builds title and description snippets, and the Schema Markup Generator produces valid JSON-LD.