[header] How to construct a perpendicular line to a given line through point on the line [title] Line up the given line and the protractor. [step] Set the protractor's origin hole over the line's given point. Align the protractor so that its base line sits exactly over the given line. 
[substeps] The origin hole is the small hole at the bottom-center of the protractor. The base line is the line on the bottom of the protractor marking 180/0 degrees.