Having passed the rope through the clock and with the counterweight in position, cut the rope to the desired length, with the clock fully wound.
A counterweight should be a couple of inches from the floor. The rope normally contains an inner core. Pull the outer core back and knot the core securely. This will be the part of the join that holds the weight. The outer sheath is then replaced and trimmed unevenly. That is, with a some strands longer than others. The idea is to avoid a clean line at the join.
With extra strong thread sew the outer sheath to the inner core. This will result in a join with no bulge.