Runge-Kutta 4 in polar coordinates

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How is the Runge-Kutta method implemented on this differential equation:

$$ \frac{d^2 \theta}{dt} = -\frac{g}{l} \theta $$

(pendulum motion) which is in polar coordinates?

Let: $c = \frac{g}{l}$

So the first step is:

$$a1 = y_2 dt $$ $$b1 = -c y_1 dt $$

Is this correct? How do I continue from here?