the shadow begins at the far left corner of the cube, then on the middle corner a line connects to the light point. you can decide where the shadow line will meet the middle line. then from there it goes to the far right corner. (I accidentally added an extra line) The horizon part is quite simple, add lines from all the visible corners and make them touch the h. point. since this is a cube, you will make a right angle to cut off the lines. version 1 explains it, on version 2 i will draw it without all the points and stuff.