how to construct a line parallel to a given line AB?

above AB put a dot 

then from the dot draw a line little bit slanting

take a compas with pencil

draw a semi-circle from AB to the line and from another side also

then measure the angle type of part with compass and with that measure

the point which is touching the line with second semi-circle cut an arc

join the dot with cutted arc and you will find the parallel line