Purchase the SplitLine (its a splitable, double cord) replacement line instead of the singe line cord much easier. The manual is available for free as a pdf from the bolens web site - you will need the serial number and part number off your unit.
If you want to use the traditional single cord line, you have to take about 20 feet of line and fold it in two (not cut). Place each open end into the thw two small holes on the side of the spool and wind them around the spool making sure they do not cross over each other or you will have problems when you try to let more line out. The manual recommends using your index finger to guide the two pieces of line around the spool (by keeping your finger in the middle of the two pieces).
Patrick, August 2007