To construct a perpendicular to a line (AB) from an external point (C),

1. draw an arc with any radius that has center C and intersects the line AB in two points.

2. Call these points D and E.

3. Using D as center, and any radius, construct an arc.

4. Using E as center and the same radius as the the arc with center D, draw another arc.

5. Where the arcs intersect, put a point F.

6. Draw line CF. Line CF is the perpendicular bisector of line AB.

Alternately, instead of continuing with steps 3-6 on step 2, you could draw the perpendicular bisector of DE.

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