The general formula is R=E/I
Stated a bit more specifically as (Vs-Vf)/Id

Where:
Vs is your supply voltage, for example 9 for a 9V battery
Vf is is the forward voltage of all your LED's wired in series, for example 3 LED's with a forward voltage of 2.5 would be 7.5 total
Id is the rated current of the LED in amps (typically around 20mA, so 0.02)

Thus, for the example values (9-7.5)/0.02 would give 75 ohms, so pick the next larger standard size (82 ohms in this case). Basically the (Vs-Vf) quantity is the voltage "left over" after the diodes have eaten their share that must be dissipated.

To calculate the power rating of the resistor, use P=E^2/R where E is that same "left over" voltage, and R is the ohm value of the resistor actually used. For the example values, 1.5^2/82 = 0.027 amps, so an eighth watt resistor would be plenty.

Another example: 24 volt supply, 2 LED's 3V 30mA:
R = (24-6)/.03 = 600 (so use 680)
P = (18^2/680) = 0.476 (so use a half watt resistor)