In this form of trataka the attention is focused only on an internal image. It is more difficult than outer trataka practised alone, or outer trataka combined with inner trataka.
Choice of image
This should be the same as the outer object that you have already chosen for trataka stages 1 and 2. If you are happy to visualize a candle flame then continue to do so. If you feel attraction for the symbol Aum then use this as your inner-image. The important thing is that you are able to internally visualize the object of your choice without too much effort.
Location of the eyebrow centre
Throughout the practice the inner image has to be visualized and projected at the eyebrow centre. This is far easier if one can feel some kind of sensation on the skin at that point. To attain this, lick one finger just prior to commencing the practice and apply a dot at the eyebrow centre. This will help to initially locate the correct position. However, though this method does help, the water will soon evaporate. For this reason it is far better to apply a little mercury, menthol, camphor or tiger balm, which will give a definite sensation at the eyebrow centre that will last throughout the entire duration of the practice. If the reader can think of a better alternative then that should be used instead.
Of course, if you can easily fix the position of the eyebrow centre then it is not necessary to adopt any means to produce a slight pressure or sensation.