The short answer
Fill the tank, inflate the tyres slightly above normal, connect a trickle charger and do not apply the handbrake if it will be months.
- Clean it inside and out and make sure it is dry. Damp plus months equals mould and corrosion.
- Fill the fuel tank, which reduces the air space and therefore condensation inside it.
- Inflate tyres slightly above the normal pressure to reduce flat-spotting.
- Connect a trickle charger or battery conditioner, or disconnect the battery entirely.
- Leave it in gear with chocks rather than on the handbrake for long storage, because the pads can seize to the discs.
- Declare SORN if it will be off the road, which stops the tax and insurance requirement.
What will ruin it
Never run an engine in a closed garage, not even briefly, and not to "keep it ticking over". Carbon monoxide is odourless and it kills people in exactly this situation.
Why it works
Cars deteriorate from standing through four mechanisms: battery discharge, corrosion from damp, seized brakes and perished rubber. Each item on the list addresses one of them.
Gas work requires Gas Safe registration by law and most electrical work is notifiable. Where a tip says to call somebody, that is the advice. Full disclaimer.