Step 1: First we have to write polynomials in descending order.
$$ \begin{aligned} P(x) &= 6d^3+4d^2-d \\ Q(x) &= 5d^2-4d+12 \\ \end{aligned} $$Step 2: Write second polynomial below the first one. Making sure like terms are lined up.
$$ \begin{array}{lrrrr} & 6d^3 & 4d^2 & -d & \\ \color{red}{ - } & & 5d^2 & -4d & 12 \\ \hline \end{array} $$Step 3: Change ALL the signs in the second row.
$$ \begin{array}{lrrrr} & 6d^3 & 4d^2 & -d & \\ \color{red}{ - } & & -5d^2 & 4d & -12 \\ \hline \end{array} $$Step 4: Combine like terms (add each column):
$$ \begin{array}{lrrrr} & 6d^3 & 4d^2 & -d & \\ \color{red}{ - } & & -5d^2 & 4d & -12 \\ \hline & 6d^3 & 9d^2 & -5d & 12 \end{array} $$