First - the planning.
Are you going to liveaboard
and sail blue water
and high latitiudes? Or coast saunter and laze on the decks in the sunshine? Are you a geek or a traditionalist? These will effect the types of systems you need and (crucial) the amount of power you expect to generate on board. If you want big batteries and short type power, then you need an inverter
. If you reckon that mini inverters can serve then you may not want large amounts of 120/240 and don't need a clever big box, but may still want very big domestic batteries and relevant charging gear
. Then you can start thinking about the wiring
Don't forget to record
what you do and where you hid things, and the spares you need!