When you're working with molds, the resin creeps up the sides a bit, so your top edges will be a bit higher than the middle. This enhances the beautiful reflections around the sides of the piece - the higher the sides, the more dramatic the reflections you'll see.
Sometimes you might want them lower, or smoother. There are two ways to do this:
You can simply trim around the edges and corners using small, sharp scissors, or you can sand them down using fine-grit (600+) wet/dry sandpaper you'll find at any hardware store. Before you begin sanding, you might want to use your scissors to trim the edges and corners down. Then, wet the sandpaper, place your piece face-down on the paper, and rub it around in a circle until you achieve the look you like. Just be careful to stop before you begin to scratch the face of your piece. Then dry off your piece using a paper towel or tissue.
Another fun technique to make your pieces smooth and rounded, is Doming-Up (see How-To Video in our Project Center), which is adding a domed layer to your cast pieces. You can embed anything into this second layer, to create beautiful dimensional effects!