Actually the most simple way. Park it and put a Battery Tender or similar on it.
That is what we do with our cars that don't get driven for the winter. Nothing special. And no problems for 11 years on the M Roadster, 8 years on the M3 LTW (and that can sit for a year or more without being driven).
'95 Alpine M3 LTW
'00 Dakar M Roadster
'02 Topaz M3
Red/White SRF #4 (Chassis 561)