Start by drawing simple shapes.
Decide which direction the 3-D space will go (arrow)
Find all the corners closest to that direction (I marked mine with dots for teaching purpose)
Draw a line in that direction, as long as you would like your cube.
Keeping the same diagonal angle draw a line extending from the next 2 corners.
Line up your ruler with the top horizontal line of the square.
Keeping the same angle, push the ruler up to the top of the box and draw a line.
Now line your ruler up with the vertical line of the square.
Keeping the same angle now move the ruler over to meet with the corner of the horizontal line you just drew.
Erase extra line
Creating a tent
Follow the diagonal
Now a triangle to pyramid
Connect top corner to bottom line to create a pyramid
Follow the same angle for all the lines
Follow the sides of the shape
Another odd shape
All extended lines are at the same angle
Matching vertical lines
Done now erase unwanted lines