Step 1: First we have to write polynomials in descending order.
$$ \begin{aligned} P(x) &= -8x^2+2x \\ Q(x) &= 5x^4-3x^2 \\ \end{aligned} $$Step 2: Write second polynomial below the first one. Making sure like terms are lined up.
$$ \begin{array}{lrrr} & & -8x^2 & 2x \\ \color{red}{ - } & 5x^4 & -3x^2 & \\ \hline \end{array} $$Step 3: Change ALL the signs in the second row.
$$ \begin{array}{lrrr} & & -8x^2 & 2x \\ \color{red}{ - } & -5x^4 & 3x^2 & \\ \hline \end{array} $$Step 4: Combine like terms (add each column):
$$ \begin{array}{lrrr} & & -8x^2 & 2x \\ \color{red}{ - } & -5x^4 & 3x^2 & \\ \hline & 5x^4 & -11x^2 & 2x \end{array} $$