A similar procedure to the passenger side but before removing inner boot panel:-
wait for sat nav unit to power down, disconnect battery, remove sat nav unit, then fold back inner boot panel, remove audio amplifier with sat nav mounting holder to expose the sunroof drain.
A bit more involved but straight forward – for more detailed instructions follow information in the Haynes manual.
Also important to follow instructions that come with the kit …… see https://mgzt.info/2021/03/09/mg-zt-rover-75-sunroof-drain-repair-kit
More posts as a tackle each of these areas.