This is a simple circuit based on transistor 2N 2222A and some resistors, there are 2 LED s and when one is on other will be off and this will be repeated in equal intervals of time

Step 1: Parts List

2 x 2N2222 transistor

2 x 10k resistor (brown black orange)

2 x 390 ohm resistor( orange white brown)

2 x 100uf electrolytic capacitor

1 x blue LED (or other colors u like)

1 x red LED (or other colors u like)

battery ( 3 - 9v)

switch

Step 2: Circuit Diagram

complete the circuit according to the circuit diagram,here i have used a common circuit board, there is a simple way to find the polarity of the LED ie, by looking for the flat part in the diameter of the LED the leg in that side will be negative

Step 3: Conclusion

use safety scissors to decrease the length of the legs of components, do not use more than 9v which will make the transistor hot and some times burned,use alligator clips while soldering the transistors which will reduce the heat of the transistor thus saving it,comment if u have any doubts