Step 1: First we have to write polynomials in descending order.
$$ \begin{aligned} P(x) &= x^5+x^3 \\ Q(x) &= 6x^5-x^3+6x \\ \end{aligned} $$Step 2: Write second polynomial below the first one. Making sure like terms are lined up.
$$ \begin{array}{lrrr} & x^5 & x^3 & \\ \color{blue}{ + } & 6x^5 & -x^3 & 6x \\ \hline \end{array} $$Step 3: Combine like terms (add each column):
$$ \begin{array}{lrrr} & x^5 & x^3 & \\ \color{blue}{ + } & 6x^5 & -x^3 & 6x \\ \hline & 7x^5 & & 6x \end{array} $$