The top surface is flattened to butt up against the grip. It can be a single piece or consist of several short cylinders butted end to end, made from different materials and of slightly different diameters and shapes for appearance and comfort. The top butts against the pommel. It can be organic or metal. This conforms to the top surface of the pommel, and must be at least 1 cm diameter, or any shape that looks good on the pommel.
Steel is probably best for small washers, or bronze or brass for large fancy ones.
It must have a central hole that fits very tightly around the tang. This important, or later on peening will be impossible, and you wouldn't want that. Slide all the pieces over the tang, testing the fit and getting everything as tight as possible with repeated filing. The metal washer is the most crucial fit. Once everything just fits over the tang nice and tightly do a final assembly, and mark the tang about 3 to 5 mm above the end of the metal washer.