How to find the particular integral? The question is to find the particular integral of the expression 
$$ (D^3+1)y = cos(2x-1) $$
I’m not sure how to go about it. How do I find the PI for this expression?

$\require{extpfeil}\Newextarrow{\xRightarrow}{5,5}{0x21D2}$This is an interesting differential equation because it yields some really nice graphs when some initial $y(0),y'(0),y''(0)$ are sampled.
As for the solution, first solve the homogenous problem:
$$y''' + y = 0 \xRightarrow{y = e^\lambda}  \lambda^3 e^\lambda + e^\lambda = 0 \Rightarrow  \lambda = \begin{cases} -1, \\ \dfrac{1}{2} + \dfrac{i\sqrt{3}}{2}, \\ \dfrac{1}{2} - \dfrac{i\sqrt{3}}{2}.\end{cases}$$
Thus, the general solution is:
\begin{align*}
y_g(x) &= c_1e^{-x} + c_2 e^{x\left(\frac{1}{2} + \frac{i\sqrt{3}}{2}\right)} + c_3 e^{x\left(\frac{1}{2} - \frac{i\sqrt{3}}{2}\right)} \\ &= c_1 e^{-x} + c_2 e^{x/2}\sin\left(\frac{\sqrt{3}}{2}x\right) + c_3e^{x/2} \cos\left(\frac{\sqrt{3}}{2}x\right).
\end{align*}
For the particular solution (aka your particular integral), based on the expression of the given differential, as Ninad Munshi mentioned in the comments, one can "guess" the expression:
$$y_p(x) = PI = A\cos(2x-1) + B\sin(2x-1).$$
Substitute then in the initial expression and calculate the values of $A$ and $B$.
Finally, the interesting part that I promised:
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