I assume the mast is up..... the easiest way to fish a line from scratch is to get yourself some sash chain or beaded chain, long enough to run down the mast, and a small enough lead sinker or sinkers to fit through the block in the mast. carefully feed the chain with sinkers attached down through the mast and let gravity pull it down to the base, use a coathanger with a hook bent in the end to snag the chain and pull it out through the exit at the base or side of the mast, attach your new halyard and pull it up. you can also use conventional fish tapes but they're hard to keep clear of the already run lines.