First fill a bucket with sea water and a hand full of rock salt. No more or you will have herring jerky. The salt will not only toughen the herring but help them last longer. Laying the herring down belly towards you, tail on your left side I cut behind the gill plate, knife close to 45 degree angle and blade straight up and down until you reach the spine. Then blade layed over and towards the tail for the rest of the cut. Always clean any remaining guts.
I use 3/o 4/o most of the time. The trailing hook I hook inside the cavity then out the side and let it dangle/trail. The front hook inside the cavity through the side and sink into the back near the dorsel. The perfect look to me is a herring that will swim and flop in a big slow circle. Looks like a wounded fish trying to swim.
Sometimes you need to readjust the front hook to get it to swim right even maybe leaving the front hook dangling as well.
Oh and the salt brine you can either cut em before you put them in or after but this brine will keep herring even into the next day most of the time.