Ideally one should be aware of the following two things while practising Aum chanting:
The sound, whether mental or verbal;
Reflection on the meaning of Aum. To do the two at the same time may be a little difficult at first, but if you can do this you will achieve the best results. If you wish you can be aware of one of the following during your practice: either the sound combined with its meaning; the sound combined with concentration on the symbol; or concentration on the symbol combined with reflection on its meaning. Choose for yourself through experimentation, or contact a competent person for advice as to what is most suitable for you.
If you simultaneously practise trataka on Aum, then the image can be external or internal, depending upon your capacity of visualization. The best is internal concentration on the symbol of Aum, but it is a little difficult for beginners. It is difficult to hold a steady image for a long period of time.
If you concentrate on an external symbol of Aum you can use any that are available, or one that you may feel inspired to draw for yourself. It should not be too large, for the aim is to focus the awareness on a limited field of attention. You can alternate external trataka with internal trataka, while simultaneously practising Aum chanting.
If you concentrate on an internal image of the symbol of Aum, then you can create it in the space in front of the closed eyes or you can create it at the heart centre as suggested by the Dhyanabindu Upanishad: “Concentrate on Aum at the heart centre as though it is a candle flame the same size as your thumb.” Make sure that you continue Aum chanting with awareness.
It is not compulsory to practise trataka on the symbol of Aum. If you wish you can remain aware only of the sound. This alone is very powerful and will quickly soothe away the tensions of the mind. This method is particularly useful for those people who have trouble practising trataka.