Been there three times in the last five months (don´t ask!)

A lot of paper towels and dish towels.
Get as much fluid out of the reservoir as possible before removing.
Open a caliper bleeder (and pump pedal) to let, more than all!, fluid
from the main cylinder empty elsewhere, but not in/on your interior!

Put rubber mat on floor. Put paper towels on the cross bream below the
brake booster. Put them on the brake booster.
Put them everywhere.

The mechanical aspect is straight forward.
Replace gasket between cylinder and booster.

Refilling:
Put towels around the reservoir opening.
Don´t overfill, especially if you bleed with a pressure bleeder on the
reservior. Removing it will spill. Use towels when removing pressure hose.

If still in place, use the stock clear plastic foil (between reservoir
opening and the instrument cluster) as emergency trap for overflowing
fluid. Better spill on the plastic than onto cluster and its
surroundings. Additional towels help.

Final hint:
Did I mention towels?!

Regards
Raimund

Am 03.11.2021 um 18:25 schrieb dan h:
> I am a total noob and I’m going to replace my brake master cylinder this weekend. Any sage advise or grave warnings before I start?
>
>  From Dan Hoffman