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One popular hand-operated model looks like a bit like a flyswatter. Use your non-dominant hand to keep it stable and in place. Your slicer will make quick work of your tomato, slicing the whole thing all at once!  Hold the tomato slicer handle in your dominate cutting hand, with the sharp end of the blades facing the tomato. For extra leverage and stability, place your pointer finger on top of the handle where it intersects with the slicer's frame. Use a sawing motion to work the slicer's blades through the tomato. When you get about halfway through the tomato, move your non-cutting stabilizing hand to the top of the tomato so that it's not in the way of the blades. You don't want to cut yourself! Keep sawing back and forth through the tomato until you reach the cutting board. Once your reach the cutting board, you should have a perfectly sliced tomato.
Get out your tomato slicer. Put your clean tomato on a cutting board. Cut your tomato. Finish.