The “classics” of SEO to achieve as a priority
1) Make sure that the indexing of your pages is not blocked because of a NoIndex tag, and X-Robots-Tag HTTP header or an indication of non-indexing in your robot.txt file.
2) Check that all your pages have a META <title> tag, which ideally does not exceed 65 characters.
3) Write META <description> tags between 70 and 320 characters that make people want to click.
4) Your URLs must be understandable by a human and, ideally, should not exceed 115 characters.
Your content itself
5) Write the content of more than 500 words, or even longer if you can (the length of the text counts for your referencing).
6) Use the right words and rich vocabulary in your content to maximize your SEO impact (use a tool).
7) Use images in your content, taking care to fill in the <alt> and <title> tags of your visuals.
8) Take care to keep the size of your images as light as possible so as not to affect your loading time adversely.
9) “Strategy” your internal network using a tool, like Google Search Console.
Take care of HTML structuring
10) Structure your content with headings H1 – H6.
11) If you have content that reads on several pages, use the pagination tags rel = “next” and rel = “prev.”
12) If your content is suitable, do not forget to add all the possible micro-data (see schema.org).
13) Your text content should ideally represent more than 10% of your HTML code.
14) Do not use hidden links or texts (for example, white text on a white background) at the risk that Google considers the maneuver as a dishonest attempt to try to influence your ranking in search engines.
User experience also has an SEO impact
15) Sure that the website is compatible with mobile devices and avoids basic UX errors from a mobile site.
16) Make sure to keep a light page, to favor a fast loading time.
17) Correct all operating errors, and hunt for pages 404.
18) Untie any broken links that may be present on your site.
19) Add a Favicon to your site, which appears on all browsers.
20) Do not think only of search engines and seek to develop a real brand on the web to win the battle of “preference.”
SEO technical elements to take into account
21) Make sure you have an XML sitemap
22) Avoid “stupid” duplicate content because of the two variants of your site, with or without “www” (for example, http://www.monsite.com and http://monsite.com). Use a <canonical> tag to declare which option you want to see in the search results.
23) Avoid that the possible parameters of your URLs (ex: when you filter a product catalog) generate duplicate content, or cannibalize the SEO of other pages.
24) In general, avoid cloaking techniques (serving different content to search engines and users)
25) Switch to HTTPs protocol (while avoiding its SEO traps)