Be seated on a chair or on the floor with your back straight. Look at a burning candle in front of you. Direct all your attention onto the candle flame, and fix your eyes onto the flame.
Feel that your breath is coming from the candle when you breathe in, and when you breath out, it is going back into the candle. Only you and the candle flame exist, nothing else.
Then close your eyes and try to see the candle flame with your inner eye as clearly as if you had your eyes open. Do not change the image of the candle, and do not think of anything else. When you loose your concentration, start the exercise again from the beginning.