Whatever frosting you are using needs to be zig-zagged across the sheet cake/brownies from one corner to the opposing corner, dragging the spatula the full width of the sheet cake/brownies. Once you start zig-zagging, don't lift your spatula up until you have reached the opposite corner.
Next you can use the tip of your spatula or a butter knife and drag it back and forth through the zig-zag that you made. You need to switch directions every time you drag it through. Example: one line would go from top to bottom, then the next line would go from bottom to top. Space lines as close or wide as you like; I typically do at least 1-inch apart. (*See video!) Continue dragging the knife through, switching directions until you come all the way across the sheet cake/brownies. This creates a rippled/marbled effect. NOTE: If your icing is wet, warm or really drippy, wait until it hardens to slice your dessert. Enjoy!