Another image
This image might be clearer. We rigged ours a little different though. Our boom came with quite an array of hardware installed by previous owners and I wasn't exactly sure what it all was intended for. We ended up rigging a single reef line that ran from a cleat on the forward starboard side of boom, up through the forward reef cringle, down to the boom port side, turned aft and ran along the boom, turned up through a block, through the aft reef cringle, down through a starboard aft block, then back to a cleat on the boom foreward. We added a boom topping lift and this system seems to work OK though we've only used it once. Need more practice. Without the topping lift it might be better to rig it like the image shows. That way you could reef down the aft cringle (hiking up the boom end) then lower the main halyard and deal with the forward cringle and tack etc.John