Use a sharp large knife. A sushi roll is usually cut into 8 even sizes, so either start at the center to get it right or envision when to cut each slice in advance. With a clean stroke cut each slice without moving the slices too much (or else the mayonnaise will get smudged). We recommend cleaning your knife with some water after each slice has been cut, or else it will start to stick and it will get harder to cut slices.