Turn off the car and all accessories
Set the multimeter to DC volts
Select a 20V range if the meter is not auto-ranging
Connect the red probe to the battery positive terminal
Connect the black probe to the battery negative terminal
Read the voltage on the display
Check for about 12.6V or higher for a fully charged battery
Check for about 12.4V for a partially charged battery
Check for about 12.2V or lower for a weak battery
Start the car and read the voltage again
Check for about 13.7V to 14.7V with the engine running
Remove the probes after testing
Clean the battery terminals if the reading seems inconsistent