Step 1: Write second polynomial below the first one. Making sure like terms are lined up.
$$ \begin{array}{lrrrrr} & 6q^4 & -q^3 & 9q^2 & 6q & 4 \\ \color{blue}{ + } & 6q^4 & 7q^3 & 2q^2 & -3q & \\ \hline \end{array} $$Step 2: Combine like terms (add each column):
$$ \begin{array}{lrrrrr} & 6q^4 & -q^3 & 9q^2 & 6q & 4 \\ \color{blue}{ + } & 6q^4 & 7q^3 & 2q^2 & -3q & \\ \hline & 12q^4 & 6q^3 & 11q^2 & 3q & 4 \end{array} $$