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