On the positive wire (probably red) from the battery, cut the wire. the ringed end should be on the end that goes on to the airplane. The Non-Ringed end should be connected to the cut end that leads back to the battery.

Important. do this splice in the wires that stay with the airplane not in the ones that stay with the battery. Otherwise you will wont be able to charge your batteries. Also if you have a way to charge the batteries in the plane (for example a solar panel,) you will need to connect these diodes between the charger and the airplane. (not between the charger and the battery.