You as a UK Passport holder need to do nothing, your girlfriend, however, needs to apply for a Schengen Visa ideally for the Port of Arrival country. It is fairly simple, having done it several times for a Phillipino girlfriend.
Of course, now as of January 1, 2021, the United Kingdom is officially no longer a member of the European Union and its relationship to the Schengen Agreement remains unchanged. The Schengen visa does not however give permission to travel to the UK