I'm pretty sure this is how you want to route the mainsheet; if you have the stock configuration (double block w/becket at the traveler; three mid-boom blocks).
Starting from the becket go trough the 2'nd (middle) block. Back down through a sheave on the double block then back up to the 3'rd (aftmost) block. Back down through the second sheave on the double block then up to the 1'st (forward) block. Now go through the block that is mounted near the gooseneck and down to a deck mounted block that will route the sheet back to the winch.
If there is no block at the gooseneck then there should be a block directly beneath the 1'st block on the deck and a second block on the deck that routes the line back to the cockpit.