First take a scale and make a line down measuring up to 10 centimetres.
Then label one side as A and another one at the opposite as B.
Then measure 10cm at your compass while letting the point of the compass stay at point A.
Then letting it stay at A make a arc upward and another below.
Then put your compass at B again taking the same measurement of 10 cm.
Then putting the compass at point B make an arc to cross the previous arcs .
Then make a line between the center of two arcs making a perpendicular.
