On kitchen knives I use the flat platen. I say kitchen knives because mine generally have a long, fairly straight spine. I hold the knife vertical (tip down) and begin with the spine flat to get my bearings. Then I begin twisting the knife left, right, left, right increasing the angle as the corners wear away. This gives me options. I can just knock off the corners, or I can make the spine completely round. I used to use the slack belt, but I had trouble getting both sides even and not getting dips and waves.
I've seen YouTube videos of people using a waterstone that had been dished with a round file (or maybe the spines of many knives which had gone before) and sawing the spine back and forth.