Normal leds (yellow, green, orange and red) uses 2 volt @ 20mA
Blue and white leds uses 3.5 volt @ 20mA
(no high bright leds counted, those will consume 50mA or more)
The resistor has to take care of the rest of the voltage. As example a green led on 12 volt needs 2 volt for the led and so 10 volt for the resistor.
R=U/I so 10/0.02 is 500 ohms. 500 is not in the standard E12 list, the best choice is 470 or 560 ohms. Watage is UxI so 10x0.02 = 0.2 watt, normal resistors are .25 watt
With my pitboard I used 2 leds and one resistor in a row at each side.
2x blue leds makes approx 7 volt so the resistor takes 5 volt from 12v.
5/0.02=250ohm, it is not listed in the E12 list so I used 270 ohms.
You can not use more leds in in serial than the voltage supplied, be sure you have to get some voltage over the resistor.
You can use a current from 10mA up to 30mA so you have some play with the resistors, to high current will result in a short life and to low current will give less light.
Be sure the resistor has enough watage, if not it will burn or have a short life.