This is a two-step process, requiring a 2-inch paring knife and a 12-inch serrated knife. This method produces level layers, the thickness of which will always be precisely what you want them to be.
Set the cake, right side up, on your cutting surface; this gives you the most level base to work from. Place your hand on top of the cake. Then bend over the side of the cake slightly until your eyes are just above the level of the cake; this gives you the best perspective for a straight, even cut. With the 2-inch paring knife, make an incision all around the cake about 1 inch deep where you wish to split it. Move the paring knife in and out in a sawing motion, simultaneously rotating the cake with your other hand until you have circumnavigated the entire cake. Be careful to maintain the incision at the same level all around the layer.
Now place your 12-inch serrated knife, blade side in, straight into the precut path at one edge of the cake. Rotate the cake as you cut through the entire layer, pushing it into the knife while simultaneously sawing back and forth.