After drawing the square on the support line, find the vanishing and depth points on the horizon line.
By the axis (vanishing point of the parallel perspective), make the inclination of the horizon line sketching the support line parallel to it. Transfer all vanishing and depth points to the fake horizon line.
From the vertex of the square drawn on the support line, sketch a line parallel to the view point line.
In any point of this line, sketch the sharp corner of the inclined cube, always paying attention to form an angle of 90º with the fake support line.