Step 1: First we have to write polynomials in descending order.
$$ \begin{aligned} P(x) &= 10x^4+x^3-2x^2 \\ Q(x) &= 3x^4+4x^3+8x^2 \\ \end{aligned} $$Step 2: Write second polynomial below the first one. Making sure like terms are lined up.
$$ \begin{array}{lrrr} & 10x^4 & x^3 & -2x^2 \\ \color{blue}{ + } & 3x^4 & 4x^3 & 8x^2 \\ \hline \end{array} $$Step 3: Combine like terms (add each column):
$$ \begin{array}{lrrr} & 10x^4 & x^3 & -2x^2 \\ \color{blue}{ + } & 3x^4 & 4x^3 & 8x^2 \\ \hline & 13x^4 & 5x^3 & 6x^2 \end{array} $$