To conduct a factory reset when Android won't boot you'll need to enter the device's recovery. How to get into the recovery varies from device to device, so you will have to search Google for the correct method of entering the device's recovery mode. Usually, power and volume up works, but no guarantees. In any event, once in the recovery you should have an option to perform a factory reset. You'll need to select that option by using the volume up and down buttons to move between menu selections and pressing the power button to activate the chosen selection. Then simply sit back and be patient.