Step 1: First we have to write polynomials in descending order.
$$ \begin{aligned} P(x) &= 8m^4+m^3+6m \\ Q(x) &= -2m^4+7m^3+6m \\ \end{aligned} $$Step 2: Write second polynomial below the first one. Making sure like terms are lined up.
$$ \begin{array}{lrrr} & 8m^4 & m^3 & 6m \\ \color{blue}{ + } & -2m^4 & 7m^3 & 6m \\ \hline \end{array} $$Step 3: Combine like terms (add each column):
$$ \begin{array}{lrrr} & 8m^4 & m^3 & 6m \\ \color{blue}{ + } & -2m^4 & 7m^3 & 6m \\ \hline & 6m^4 & 8m^3 & 12m \end{array} $$