To tension the shrouds, you twist a threaded turn-buckle a couple of turns at a time, alternating on each side and sighting up the mast each time to make sure things are straight and true. Befre we started, we went around the inside of the boat, tightening the bolts on the chainplates, which transfer the loads from the mast shrouds to the hull, via various bulkheads.
It took about two hours to undo all the wires that prevent the turn-buckles from losing their adjustment, tension all 10 wires, and re-affix the locking wires. Thanks to John we now have a nice evenly tensioned rig with between 11% and 13% of breaking strain on each wire - more tension being on the back and fore stay as well as the primary shrouds (the ones that go all the way to the top of the mast). Hopefully this will get us across the Pacific and back.