You need a 50 foot nylon electrical fish, basicaly a spool with an nylon thread similar to what is on your whipper snipper. Just feed it down from the top and fish it out at your exit point. not very difficult.
You can find them at Princess Auto or similar.
Gary
Unfortunately, it's not quite that easy. There is a very good possibility that you will cross or even wrap the halyards around each other by running them individually.
I would suggest that you remove the mast head (if possible) and get a very strong flashlight or a mirror if the sun is shining to see what's going on inside the mast. Make up a little flat piece of something. Like a piece of plastic shaped like a V that is just large enough that it can't rotate inside the mast but can still run free. Attach a messenger for each halyard to this piece. Run a single messenger from the top of the mast to the bottom with the fish tape and with that, pull the V down and fish each messenger out at it's exit. After all the messengers are run, pull them tight a look to make sure that they are running fair.
Alternatively, if you only have two or three halyards, You may want to just wait until the mast is stepped. Send a man aloft with a weighted messenger and while heeling the boat significantly to the side that the sheave and exit is on, have him feed the messenger down. Heeling the boat will keep the messenger sliding down the side of the mast preventing it from fouling the others. Repeat the process for the other side.
Remember to snug up the previous messenger before running the next.
Either way you choose to do it, I would suggest that you not replace the messengers with the halyards until you have run them all and have ascertained that they are running fair.
Good luck.