Remove the blind from its mounting brackets. In most cases, you will see a cover on the bracket. Slide the cover off the bracket. Pull the head rail of the blind free from the brackets. In other cases, the head rail clips onto the bracket. In this instance, lift up on the head rail to remove it from the bracket.
Spread out the blind on a table.
Thread the broken string back through the holes in each slat until the ends of the drawstring meet.
Tie a small knot with the ends. Pull the knot tight enough so that it will fit through the holes in the slats and so that the slats can move without trouble.
Trim excess string off the knot with scissors.
Place the blind back onto the mounting brackets and secure it.