Gauss Lemma (Number Theory)

Use Gauss Lemma (Number theory) to calculate the Legendre Symbol $\displaystyle (\frac{6}{13})$.

I know how to use Gauss Lemma. However we use the book: Ireland and Rosen. They define Gauss Lemma as:

$\displaystyle (\frac{a}{p})=(-1)^n$. They say: Let $\displaystyle \pm m_t$ be the least residue of $\displaystyle ta$, where $\displaystyle m_t$ is positive. As $\displaystyle t$ ranges between 1 and $\displaystyle \frac{(p-1)}{2}$, n is the number of minus signs that occur in this way. I don't understand how to use this form of Gauss's Lemma