You can use styrofoam balls for everything from making Christmas tree ornaments to packing materials. If you are using these balls for crafts, then you may want a size that is not easily purchased at a craft store. In this event, you may elect to make your own styrofoam balls by carving them from a block of styrofoam.
Decide on the measurements of your styrofoam ball. You will need to determine how long you want the diameter (length across the centre of the ball) to be.
Cut out a cube from the styrofoam block. Its length, height, and width should all be the same as your intended diameter.
Carve the rough shape of your styrofoam ball. Start out by shaving off the corners of the cube so that it becomes more rounded. You can use the serrated knife for this. Work slowly to ensure that you do not cut too much off at once. Take the same amount off each corner, then evaluate the shape before removing more. As you start taking off less and less styrofoam, you may want to switch to the butter knife, which will let you rub off layers of styrofoam rather than slicing them.
Sand your styrofoam ball. Once your ball is fully round, you can finish it off by sanding it lightly with the sandpaper. Do not press hard on the ball or you will just rub off more styrofoam. Use gentle pressure to insure that you smooth the edges rather than distorting the shape of your ball.
Be careful as you carve with the knives until you are used to the way they react to your styrofoam. You do not want to slip and cut yourself.