The existence of a series resistor is absolutely vital for a LED.
Itīs required for each branch containing one or more LEDīs connected together serially.
Maybe, these following words are of any use, too.
If you buy LEDīs, ask the dealer about their voltage and current (or see the according data sheet), because thatīs what you need to calculate the resistor.
This resistor is calculated like this:
Voltage of resistor = 12V - Usum
...whereas "Usum" is the summary of voltage of LEDīs, if connected together serially. (Btw., if thereīs another voltage in a branch, you will have to replace the 12V by the total voltage of that branch)
Letīs call this voltage of the resistor "Ures".
Total current = same than a single LED needs, if connected serially.
Letīs call this "Itot"
Now, the value of the resistor "R" in Ohm is...
R = Ures / Itot
If thereīs no such a resistor available, just take the next one above the calculated value.
A 1/4 Watt type should do, usually.
Donīt forget to add a switch to whatever you want to build.
Your battery will be very grateful to you.