If the drawcord isn't stretched out or missing any part, use the old drawcord as a reference to measure and cut the appropriate length for a new one.

Alternatively, to measure and cut a new drawcord without the old one:

Measure the length of the hem.

Add the distance the cord travels to and from any cord locks.

Add 2 inches (~51 mm) to account for a 1 inch (~25 mm) seam allowance on each end.

Add an additional 1 inch (~25 mm) if the drawcord end knots were cut in Step 2.

Use a lighter to singe the cut ends of the drawcord.

Discard broken pieces responsibly.