Corncob Jelly Mash 12 red corn cobs. Chop into 1-inch long pieces. Add water to cover and boil for 30 minutes. Strain; measure out 3 cups liquid. Add 3 cups sugar and 1 box Sure-Jell®. Follow directions for making jelly on Sure-Jell® box, except boil for 5 minutes instead of time stated. Pour into hot, sterile jelly glasses and seal.