Collection Schematic

For a lower parts count than the 2 transistor multivibrators, 2 LEDs can be alternately flashed with a 555 integrated schema configured as shown in Schematic 2.  I chose the combination of a 2K2 and a 47K resistor to determine the oscillation frequency along with the 10 uF capacitor connected to pins 2 and 6. You can practically change the (R Speed) 47K value to between 10K and 100K or more. Greater resistance = lower speed. You may also wish to connect up a 100K or so potentiometer instead of the 47K resistor for a variable speed version. Additionally, the 10 uF capacitor value can be changed. Feel free to experiment.

Although, alternately flashing LEDs  is great for the beginner to electronics, the basic one ON, one OFF schema gets boring quickly. In the next section, we will try to improve the look and try to approximate a flash like a police car (within limits).