
Start with a given ray. 

Step 1 
Pick a point not on the line, about 6 cm from the endpoint of the ray.
Its exact location is not important. Label it D. 

Step 2 
Set the compasses on point D and set its width to the endpoint of the ray. 

Step 3 
Draw an arc that crosses the ray and extends over and above the ray endpoint.
(If you prefer, draw a complete circle.) 

Step 4 
Draw a diameter through D from the point where the arc crosses the ray. 

Step 5 
Draw a line from the ray's endpoint to the endpoint of the diameter line 

Step 6 
Done. The last line drawn is perpendicular to the ray. 
