We want to control both LEDs by controlling the current flowing through each of them in an alternative (one after the other) manner.
To do this, we will be switching each loop between a closed circuit and an open circuit state. This way we are turning each of the LEDs ON and OFF alternately. This can be achieved through a simple combination of capacitors and transistors.
Start by placing our two LEDs on the board (as shown on the breadboard diagram). Don't forget to add two resistors (330Ω) to protect the LEDs from too much current being delivered by the battery!