We need to construct it using ruler and compass only.
Step 1: Draw a line PQ and take a point R outside it.
Step 2: Take a point J on the line PQ and join it with R.
Step 3: Take J as a centre and draw an arc with any radius which cuts PQ at C and JR at B.
Step 4: Now with the same radius, draw an arc taking R as a centre.
Step 5: Take the measurement of BC with compass and mark an arc of the same measurement from R to cut the arc at S.
Step 6: Now join RS to make a line parallel to PR.
∠ARS = ∠BJC, hence RS ∥ PQ because of equal corresponding angles.
This concept is based on the fact that a transversal between two parallel lines creates a pair of equal corresponding angles.
Remark: This can be done by taking alternate interior angles instead of corresponding angles.