What information do you have that users are interested in searching for and how can you provide it in a way that answers their needs and helps them to move forward?
Is this something that (Google would) want to recommend to other users?
Title tags and meta descriptions
Avoid duplication in your title tags.
Aim to have a title and description under 55 and 155 characters respectively.
Aim for a word count of 300+ words.
Do NOT duplicate content
Page load times
Check the size of your images
Check for broken pages
Google Search Console: Navigate to Crawl, Crawl Errors, “Not Found”
HTTPS vs HTTP
The English Google Webmaster Central office-hours hangout, 22/02/2019 talks about interstitial (pop-up) dialogs resulting in lower SEO rankings.
an advertisement that appears while a chosen website or page is downloading.
If Google finds a persistent 302 (temporary redirect) for a long period of time, it will start treating it like a permanent redirect. (from English Google Webmaster Central office-hours hangout, 22/02/2019).
Sub-domains are effectively treated as separate sites for the purposes of SEO:
Here’s Why Your Site Structure Matters for SEO by Eric Enge: https://www.youtube.com/watch?v=xvSuaVuJuJM