I don't know of the reference you make but the principle is simple enough. You do need the three sided trough. At each end there should be an endplate with a hole that is the same diameter as the square is across the diagonals. There should be sides across the trough that hold and keep the router centered to te trough. Then it is just a matter of running the router back and forth down the trough as you slowly spin the oar handle. If you want it tapered you make one end hole higher than the other. There will be a square end on the outer end that you can trim off. The square end on the inner end can be done by hand or you can make another plate with a hole equal to the newly turned diameter and placed before the inner squared part and then turn it down the same way as the rest.
If you want to make a handhold on the end with a raised half-round bead for the the base of the palm to rest against, you can use a plunge type rounding bit of the right radius, probably 3/16 to 1/4 " radius. Just hold (or clamp) the router in place, plunge the bit, and rotate the oar to make each side of the raised half-round bead. The handle end of the oar can also be shaped in this way with appropriate bits if you are so inclined. Hope this is clear.
Someone I consider a master woodworker once told me that a master woodworker is not someone who never makes mistakes. He is someone who is able to cover them up so that no one can tell.