I've done it two ways. if you're using one of the single braid products - maffioli or Robline Racing sheet, splice a piece of 1/8" spectra into the tail to give you a "Y" split. If you're using a double braid product like Endura Braid. Pull the core out about 12" from the end, brummel it back through the cover, and whip in place. Theat will give you a core on one side, and cover on the other side of the "Y".

If you want a mainsheet with the "Y" splice already in place, PM me.