Mark the back corners of the block.
Draw 2 receding lines from the bottom vanishing point to the back corners of the block.
Draw 2 receding lines from left and right vanishing points to the back corners of the block.
The block is done now. Outline the visible parts of the block.
Divide the three closest corners into grid marks.
You can use the "post" method or "eyeball" the marks reducing the space between them as you move away from the front corer.
Connect each mark with at least two of the vanishing points.
Now, that the block is done...
Mark a cutout area.
Erase everything inside it.
Define the three visible inside surfaces - draw the three receding lines through the corners.
Pick a side in the cutout ares:
From the points, where the grid-lines stop at the edge of the cutout - draw receding lines.
Repeat the step on the other side.
Now the bottom side of the cutout is completed.
Repeat the steps on the other two surfaces of the cutout.