Your construction is incomplete. You need to drop your sun on the horizon, and draw rays from the dropped sun through all ground vertices of the cube. Intersect these rays with the rays you already have to find the shadows of the vertices.
here's an image to help you with the idea eezacque mentioned
Thanks for the extra reference material dpaint.
After eezacque told me to drop a line down from the light source to the horizon line and draw through all four corners of the box base, the lines finally intercepted and I was able to connect the dots...literally
That circle box plane is very helpful. I was wondering how the book author know where to draw the ball shadow elongated as he projected the ball's shadow on a vertical standing wall.
I swear the book was so subtle about it, it didn't even mention about drawing lines from the base of the point light source
So what about sun light rays that are parallel, do we do the same? Drop a line down from the sun to the horizon, then draw through all 4 corners of the box base?
As Eezacque pointed out, they should all go towards the same shadow vanishing point on the horizon.
If, however, you're talking about when the light source is to the side, you don't use a shadow vanishing point. You draw one arbitrary line through the top of one corner, and then the bottom of that corner, you just draw a straight line going right/left/whatever, and it'll join up with the first line. Then you copy those arbitrary lines for the other corners, and do the same straight lines at the bottom of those shapes.