bode plot, zero at 0Hz
$$H(s) = \frac{R C s}{R C s + 1}$$

If $R C s$ is much lower than 1 (i.e., $R C s \ll 1$, in Math), then you can make an approximation: $$H(s) \simeq \frac{R C s}{1}$$

Basically, the effect of the $R C s$ in the denominator becomes insignificant.

So, just pick a radian frequency well below $1 / R C$ and compute the magnitude of $H(s)$ at that point -- mark that on your plot, and draw your line rising at 20dB/decade from there.