Measure how tall your cell phone is. Multiply that by 2 and then add 1-inch to that measurement and that is how long you want to make your cell phone case. My phone is 4 1/2-inches long so I want to cut my cell phone case about 10-inches long (4 1/2 x 2 + 1).
Use a ruler to mark the cutting lines on your duct tape sheet. The piece I cut from the duct tape sheet is approximately 3 1/2" x 10". Cut it out.