After opening your sugru pack divide the sugru in small parts, depending on the number of keys you want to cover. Usually with a single Sugru pack you can cover three normal keys. Clean with alcohol the keys surface.
Divide again each part in two and place two halves on opposite sides You can both push a Sugru ball on the surface, or mold the Sugru in a cylinder and place it around the hole. I prefer this last method, since in the first case you then have to open the hole and move the paste away. Help yourself with a pen to open and keep clean the hole, then clean all the edges, push Sugru with you fingers on all the surface, especially on the edge.
I used a piece of cloth to imprint a texture on the paste surface. Here is where you can make your keys very characteristic. Depending on the fabric or the object you use, key's head will be more or less unique.