Serving notice on assured shorthold tenancy that term ends.

Tenancy Types and Management

I have a tenant on an assured shorthold tenancy agreement ending 31 December 2021. The term was 12 months. If the agreement has the following clause, how do I notify the tenant that the tenancy will end? Is it the 6a form or is written notice enough? The tenant is a nightmare and I do not wish to continue the relationship with them so I want to serve them notice now that December will be the last month on this tenancy agreement - but I'm not sure what I legally have to do/say to do this and whether this is different from serving notice for sec 21 or 8.

"Should neither party have brought the Tenancy to an end at or before the expiry of the Term, then a new tenancy from month to month will be created between the Landlord and the Tenant which will be subject to all the terms and conditions of this Agreement but will be terminable upon the Landlord giving the Tenant the notice required under the under the applicable legislation of England (the "Act")."