In doing this alot of you go the method of just buying the leds with resistors. Welp me being the tough guy I had to try it out....Well it turns out that it really isnt hard to figure and theres some great info
Here



VS = supply voltage
VL = LED voltage (usually 2V, but 4V for blue and white LEDs)
I = LED current (e.g. 20mA), this must be less than the maximum permitted