Light a candle in a comfortable & quiet room (preferably without breeze, you don't want the candle to flicker). Sit in an upright position in a chair, on the floor cross legged, or in Padmasana (Full Lotus Pose). Chose a position in which you can keep your spine straight.
If possible the flame should be at arms length and at eye level.
Begin by taking a few deep breaths. Focus your eyes on the flame. Keep your gaze without blinking for as long as you can. You may start to feel tears develop. Keep your eyes focused. If you feel you need to half close your eyes, this may help. Gaze gently at the flame, not the candle. Let your vision stay with the flame. If thoughts arise, simply acknowledge them and let them go.
When tears flow (or you feel you need to), close your eyes. You will see the after-effect of the flame. Keep your eyes closed and see the image of the flame glow behind your eyelids. Focus on the flame's image and notice how it may be changing. It may glow very brightly and then grow dim, only to glow brightly again. It may change colors and size.
Now bring your attention to the space between and just above the eyebrows to the Ajna Chakra, the third eye. You may (or may not) feel deep peace and distance from your body. Allow that feeling to permeate your entire being.
After the flame has diminished from your mind's eye, slowly open your eyes. Do not look at the flame.
This practice will take time to unfold. It won't happen overnight, but keep practicing. Regular and persistent practice of Tratak will bring greater awareness, a relaxed state of mind, enhanced brain function, and a deeper knowledge of the Self, all leading to the path to Enlightenment.