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.

Like so.

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.

Cube

Erase extra line

Completed cube

Triangle

Add lines

Creating a tent

Follow the diagonal

3D triangle

Now a triangle to pyramid

Connect top corner to bottom line to create a pyramid

Pyramid

Odd shapes

Follow the same angle for all the lines

Follow the sides of the shape

Erase

Another odd shape

All extended lines are at the same angle

Matching vertical lines

Done now erase unwanted lines