Step 1: First we have to write polynomials in descending order.
$$ \begin{aligned} P(x) &= -5x^5-11x+12 \\ Q(x) &= -4x^5+4x^4+2x^3+8x-1 \\ \end{aligned} $$Step 2: Write second polynomial below the first one. Making sure like terms are lined up.
$$ \begin{array}{lrrrrr} & -5x^5 & & & -11x & 12 \\ \color{red}{ - } & -4x^5 & 4x^4 & 2x^3 & 8x & -1 \\ \hline \end{array} $$Step 3: Change ALL the signs in the second row.
$$ \begin{array}{lrrrrr} & -5x^5 & & & -11x & 12 \\ \color{red}{ - } & 4x^5 & -4x^4 & -2x^3 & -8x & 1 \\ \hline \end{array} $$Step 4: Combine like terms (add each column):
$$ \begin{array}{lrrrrr} & -5x^5 & & & -11x & 12 \\ \color{red}{ - } & 4x^5 & -4x^4 & -2x^3 & -8x & 1 \\ \hline & -9x^5 & 4x^4 & 2x^3 & -3x & 11 \end{array} $$