My suggestion, using a variable resistor (range from about 100ohm to 1Kohm or higher). Connect your 12V battery, variable resistor and a LED in series (before this, please make sure that the variable resistor is set to it's MAX resistance). After the connection, slowly turn the variable resistor to decrease the resistance until u get the desired brightness of the LED.
Without changing anything further more, measure the Voltage across the LED, the value u get is the typical forward voltage of the LED.
Also, measure the current passing through this connection, the value u get is the typical forward current.
u can use this method for ur further LED cct design to determine the suitable series resistor value for ur desired LED brightness.