SEO Help

<link rel="alternate" href="http://example.com" hreflang="en-us" />
<link rel="alternate" hreflang="lang_code" href="url_of_page" />

https://ahrefs.com/blog/seo-meta-tags/

Meta title
<meta name="title" content="page_title">

Meta description
<meta name="description" content="page_description"/>

Meta Keyword
<meta name="keyword" content="tags"/>

Meta robots

Meta refresh redirect

Meta charset
<meta charset="UTF-8">

Meta viewport
<meta name="viewport" content="width=device-width, initial-scale=1, shrink-to-fit=no">

Meta Language
<meta name="language" content="en">

Meta Author
<meta name="author" content="author_name">

image tag add alt="" attribute
add sitemap.xml
add robot.txt

https://support.google.com/webmasters/answer/189077?hl=en&visit_id=637199370344879974-2684573339&rd=1
<head>
  <title>Widgets, Inc</title>
  <link rel="alternate" hreflang="en-gb"
        href="http://en-gb.example.com/page.html" />
  <link rel="alternate" hreflang="en-us"
        href="http://en-us.example.com/page.html" />
  <link rel="alternate" hreflang="en"
        href="http://en.example.com/page.html" />
  <link rel="alternate" hreflang="de"
        href="http://de.example.com/page.html" />
  <link rel="alternate" hreflang="x-default"
        href="http://www.example.com/" />
</head>