Google "Ohm's Law +LED"
There's a few good sites that will explain how to figure things out. You won't need any capacitors, but you will need good soldering skills. Also, you will have an issue with your flash rate (turn signals). That has been the single most frustrating aspect of the whole project.
I suggest that you make a test bench (12v) from an old computer power supply, and get a breadboard (or two) from Radio Shack. This is where you can tune your resistor values for the correct brightness.
Oh, forgot to answer the question:
If the Vf is 3.5V, the supply is 12V, the desired current is 20 mA, then:
12-(3.5+3.5) / .020 = 250ohms (two LEDs per resistor)
12-(3.5+3.5+3.5) / .020 = 75 ohms (three LEDs per resistor)
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