Actualy Leds are quite easy to work with.

Just remember that in case of parallel LEDs each led needs its own resitor, since electricity takes the shortest road and if both use the same resistor, one of them will work, the other won't.

In case of connecting them in series you have to make sure that they all use the same operating current.

If you have a led with an operating voltage of 3.8V and a current of 20mA (most of them are 20 mA these days) and your power supply is 5v, than the resistor has to lower the voltage by 1.2V. and the resistor should be 1.2/0.02 = 60 Ohm.

If the difference between the operating voltage is more than the reistor should be a higher value so if your power supply is 9V then the difference is 5.2 V and the resistor should be 5.2 (V)/0.02 (A)=260 Ohm to have the LED working under ideal circumstances.

If you have a 9V powersupply and your LEDS are of the 2.8V 20mA variety you can put 3 of them in series and you need 8.4 V to make them work, that is a 0.6V difference. In that case you need to put a reistor in series with the LEDs of 0.6/0.02=30 Ohm.

If you have a mixture of Leds with different operating voltage and the same current in series you just add up all the operating voltages.

for instance a 9V power supply, a green Led of 2.2v,/20mA, a red LED of 1.8 V/20mA and a yellow LED of 2.8v/20mA you can put one of each in serie and you would need 6.8 V to make them work optimal.

Thats a 2.2V difference thus the resitor in series should be 2.2/0.02 =110 Ohm, nearest is a 120 Ohm resitor.

Since you don't need to be precise , you can use the nearest resitor from the E24 table but it might be wise to take the nearest one that has the higher resitance so your LED will be working on lower than max voltage, it will be a bit dimmer, but not noticably and it will live longer.

So the rule is add the voltages for the LEDs if you put them in series , subtract that from the voltage of the powersupply and devide them by the current to determine the right resistor value.

If you multiply the voltage (V) that is dropped over the resistor by the current(A) you get the power that the resistor consumes W=V*A so you can choose a 1/8, 1/4 or 1/2 W resitor as needed

Ohm =V/A.

And that is all you need to know to figure out how to operate LEDs.