Step 1: Write second polynomial below the first one. Making sure like terms are lined up.
$$ \begin{array}{lrrrr} & & -3x^2 & 8x & \\ \color{blue}{ + } & -4x^3 & 6x^2 & 4x & 1 \\ \hline \end{array} $$Step 2: Combine like terms (add each column):
$$ \begin{array}{lrrrr} & & -3x^2 & 8x & \\ \color{blue}{ + } & -4x^3 & 6x^2 & 4x & 1 \\ \hline & -4x^3 & 3x^2 & 12x & 1 \end{array} $$