Cut slits into each end of the ruler using the hacksaw, aligning the blade in the middle of the ruler tips and aiming down the length of the ruler. Make each slit around 1 1/2 inches deep.
Hammer the nail through the ruler at the 6-inch mark -- the mid-point of the ruler. Spin the ruler around on the nail a few times so it's not too stiff.
Cut an arrow head and tail from your plastic sheet. The design is up to you but the head and tail must be broad enough to catch the wind. Fit the plastic arrow parts into the slits at either end of the ruler, securing them into place using the glue. Let them dry.
Put a washer on top of the broom handle and place the tip of the nail in the center of the washer. Hammer the nail down into the broom handle until the base of the ruler is flush with the washer. Your completed wind vane must spin easily on its axis.
Take your wind vane outside and mount it to the highest point possible. If there is a hill nearby, push the broom handle into the ground at the top of the hill. Otherwise, mount it to gutter or a fence using wire.