I use wheel nippers for keystoning. It's not as difficult as you may think. The birdbath below is my first try at keystoning.
Hold the vitreous tile in your hand and place your wheeled nippers on a slight angle on the tiles (not directly in the middle of the tile). Cut and place all the small edges together on a table this way you can see what curves you have created and if you need to change any of your angles. Depending the the angles you would like you may also want to pop in some straight cuts here and there. If you alternate between the larger and smaller side of the tile in the centre you can create bends and shapes.
Now that you have mastered keystoning try using different colours for different effects!