Step 1: Write second polynomial below the first one. Making sure like terms are lined up.
$$ \begin{array}{lrrrr} & -2x^3 & 3x^2 & -8x & -12 \\ \color{red}{ - } & & & 2x & 3 \\ \hline \end{array} $$Step 2: Change ALL the signs in the second row.
$$ \begin{array}{lrrrr} & -2x^3 & 3x^2 & -8x & -12 \\ \color{red}{ - } & & & -2x & -3 \\ \hline \end{array} $$Step 3: Combine like terms (add each column):
$$ \begin{array}{lrrrr} & -2x^3 & 3x^2 & -8x & -12 \\ \color{red}{ - } & & & -2x & -3 \\ \hline & -2x^3 & 3x^2 & -6x & -9 \end{array} $$