This is an easy little electronics project. The details of how it works can be found on Wikipedia under Multivibrator.
Below is the circuit diagram and protoboard layout which makes it easy to build for anyone with some soldiering skills and a few dollars.
I have attached two red LEDs to the outputs, where they will turn on and off alternately a few times every second. If you only connect one output you will have a single flashing LED. This project can be fine tuned to make a variety of other more complex projects.
R1, R4 – 2K2 Ω resistor
R2, R3 – 3K3 Ω resistor
Q1, Q2 – BC548 NPN transistor
C1, C2 – 100μF capacitor
D1, D2 – red LED
R5, R6 – 220 Ω resistor
Now see the final product in action…