There is a public domain Section 21 flowchart, often referenced in threads here, located at:
https://nearlylegal.co.uk/section-21-flowchart/
Landlords can make use of this to ensure they are not bamboozled by the requirements to successfully serve this.
Be aware the Government seems to have plans to abolish Section 21 in due course. And, as the page linked to says... "The usual disclaimers apply..." and "There are caveats – lots of caveats."