You need to go through the entire process of removing the old string from the spool. Push the new line into the central hole in the spindle for up to ½ inches. Weed eaters that use two lines will have two of these holes. Start to wind the string in the direction indicated on the spool. Keep going until you are ready to turn a fresh layer. If the spindle requires two lines, repeat with the second one. When you are through, clip the strings in the notch on the spool. Slot the spool in trimmer head and screw or snap the retaining cover into place.