Drawing perpendicular to a line through a point – Using ruler and set-square

Let’s take an example

1. Given a line l
   with point P marked on it
   
2. Put ruler on the line so that line is exactly above the ruler
   

3. Place set-square on top of the scale

4. Move set square so that its edge is at the point P
   
5. Draw a line along the edge of the set square, keeping set-square firm
   
   

   
   So, PQ is the line perpendicular to l