(i) Place a set-square on PQ such that one arm of
its right angle aligns along PQ
(ii) Place a ruler along the edge opposite to the right angle of the set-square.
(iii) Hold the ruler fixed. Slide the set square along the ruler till the point R touches the other arm of the set square.
(iv) Join RM along the edge through R meeting PQ at M. Then RM perpendicular PQ.