Step 1: Write second polynomial below the first one. Making sure like terms are lined up.
$$ \begin{array}{lrrr} & 8a^5 & & -4 \\ \color{blue}{ + } & 3a^5 & a & -2 \\ \hline \end{array} $$Step 2: Combine like terms (add each column):
$$ \begin{array}{lrrr} & 8a^5 & & -4 \\ \color{blue}{ + } & 3a^5 & a & -2 \\ \hline & 11a^5 & a & -6 \end{array} $$