With any luck, the pressure you applied to the nutcracker with the walnut in the correct position will cause the nut shell to begin to start to crack in half from the pointed end down towards the center of the nut. As soon as you hear or notice the seam opening, check that the nut isn't slipping in the arms of the cracker but is still lined up with the seam centered on the arms of the nutcracker. Continue to apply gently even pressure to the nut using the nutcracker to keep the seam line opening. Try to make sure you are pressing evenly on both arms, squeezing them together in your hand. If you apply too much pressure on one side you may cave in the nut where it touches the nutcracker. Use the hand not holding the nutcracker to cradle the walnut in position as you apply pressure. If the walnut shell begins to 'give' under the pressure of the nutcracker, try reversing the walnut in the nutcracker arms so that the rounded bottom of the nut is facing the top of the nutcracker. Most nuts will crack in one press with the nutcracker, but some more stubborn or larger nuts may only crack half way and then begin to give rather than open.
When you have successfully opened the walnut into two even halves, gently remove the walnut meats and clean out the inside of the walnut to prepare it for use as a miniature ornament or container.