First, measure the length of the sides of ABC and make a note of them. You need to calculate the lengths of the scale drawing triangle (call it DEF where D corresponds to A, E to B and F to C) from these by calculating 2/3 of each side. Make a note of these scaled measures and which sides of DEF they apply to.
Decide where you're going to draw triangle DEF and mark where angle D (vertex D corresponding to vertex A) is going to be. To draw side DE corresponding to AB, align the longest side of the set square with AB and place the ruler against whichever of the other two sides of the set square is going to allow you to slide the set square along the ruler so that it passes through where you marked point D. You may need to experiment until you have found the best position for set square and ruler. Draw a line from D using the longest side of the set square as a guide. Use the ruler and measure along the line according to the length of DE calculated earlier. Label the ends of the line DE. DE and AB should be parallel and DE should be scaled to 2/3 the length of AB.
Use set square and ruler in the same way to draw either DF or EF parallel to AC or BC. There's no need to draw the third side using this method because when you have drawn two sides you can simple join their ends together. However, you should check that the length of the side is as expected from your earlier measurements. If it isn't the right length you will have to repeat the exercise, with care this time!
