A locus is a pathway which is formed, with equal distance around a point. Loci is the plural.
Constructions are drawn using a compass and ruler. Remember: do not rub out your construction markings.
If you start with a point that is perpendicular to a line:
Then, open the compass so it is slightly longer than the distance from P to the line, place it on point P and put a cross on either side.
Then place the compass on point $E$ and $F$ and draw arcs:
Then draw a perpendicular line: