This is the stepbystep, printable version. If you PRINT this page, any ads will not be printed.
See also the animated version.
After doing this  Your work should look like this  

Start with a line and point R which is not on that line.  
Step 1  Place the compasses on the given external point R.  
Step 2  Set the compasses' width to a approximately 50% more than the distance to the line. The exact width does not matter.  
Step 3  Draw an arc across the line on each side of R, making sure not to adjust the compasses' width in between. Label these points P and Q  
Step 4  At this point, you can adjust the compasses' width. Recommended: leave it as is. From each point P,Q, draw an arc below the line so that the arcs cross. 

Step 5  Place a straightedge between R and the point where the arcs intersect. Draw the perpendicular line from R to the line, or beyond if you wish.  
Step 6  Done. This line is perpendicular to the first line and passes through the point R. It also bisects the segment PQ (divides it into two equal parts) 