Cut around the top of a 5-gallon bucket above the handle to create a form. Set the form on a sheet of plywood.
Fill form with leftover concrete and use a trowel to smooth out the form. Use the handle end of the trowel to tap the side of the bucket to get rid of any air bubbles that may have formed in the concrete, then use the trowel to smooth out the cement one more time. Vibrating the concrete mold helps to lessen the air bubbles.
When the concrete is set, remove form and you have a great stepping stone. It takes four days to cure the wet concrete in the mold.