An easy way to implement this is to always increment the cycle count, but only increment it again if the current instruction is an addx instruction. We update the total strength after the first increment and after both the second increment and X register update.
This time, we check whether the current cycle lies on the sprite's position before the first increment and before the second increment. We only update the X register after both increments have completed, similar to Part 1.
The core idea for this problem is relatively simple, but it's extremely easy to make off-by-one errors like updating the cycle counter after instead of before drawing the pixel.