First you must remove the globe and then the burner. Discard the old wick after removing it from the burner.  Trim the new piece of wick straight across, then apply 1/4 " of a 3" strip of masking tape to one end of the wick, and fold the tape back onto itself and wick, making a 1 1/4" long leader.  Trim the masking tape to the width of the wick, and insert it into the underside of the burner using it to guide the wick.  Once the wick is through the burner remove the tape and re-trim the wick straight across if necessary.  This method can also be used for round wick using a narrow strip of masking tape as a leader.