You should always try to avoid cutting a live electrical wire under all circumstances. But if for some reason you must cut a live electrical wire, there are ways to do it while minimising the potential for harm.
Put on your insulated gloves and insulated shoes. The shoes should have rubber soles.
Look to see if there is any conductive substance near the wire or wires that need to be cut. Specifically, if there is any moisture, such as water, clean it up prior to cutting the wires.
Cut only one wire at a time. Do so very carefully by gently clamping down on the wire with the cutters.
Wrap the open ends of the cut live wire with electrical tape to ensure you do not accidentally electrocute yourself.
If possible, always shut off the current to a wire you are going to cut. Call an electrician if you have any reservations about working with electricity.
Consider wearing goggles to protect your eyes in case of sparks. Thoroughly check to see there are no holes in your gloves or in the insulation on the tool.