Riding lawnmowers have a construction similar to that of automobiles: a gas-powered motor that has a battery-powered electric ignition. Like a car, if this battery dies, you cannot start the engine. However, you can charge or "jump-start" a lawnmower battery just as you can a car battery.
Locate the negative terminal on the lawnmower battery. Terminals are the places on the batteries where the cables connect. The negative terminal is black and usually has a "minus" (-) sign by it, while the positive terminal is red and has a "plus" (+) sign by it. Disconnect the negative contact on the lawnmower battery. This may require a screwdriver or pliers, depending on the model. Do not let the cable touch anything.
Connect the lawnmower battery and car battery with your jump leads. Connect a positive (red) clamp to the positive lawnmower battery terminal. To open a jump lead clamp, squeeze it as you would with scissors or tongs. Springs in them will make them latch on to the battery terminals. Connect a positive clamp to the positive car battery terminal. Connect a negative clamp to the negative (black) lawnmower battery terminal. Connect a negative clamp to the frame of the car.
Turn the car on, but keep it in neutral. Press the accelerator pedal. Let it run at around 2000 RPM for about two minutes.
Disconnect the jump leads. Replace the negative battery contact on the lawnmower. Start the lawnmower.
Do not attempt to start the lawnmower while the battery contacts are still connected. This can damage any computers you may have in your car. Also, do not allow any metal objects to come in contact with the terminals. Metal objects act as conductors and could bridge the connection between either the two terminals of the battery or the car frame, resulting in a potentially serious electric shock or damage to the battery.