Gaze at the picture of the Lord, your Ishta Devata for a few minutes and close your eyes. Then try to visualise the picture mentally. You will have a well-defined or clear-cut picture of the Lord. When it fades, open your eyes and gaze again. Repeat the process five or six times. You will be able to visualise clearly your Ishta Devata or tutelary deity mentally after some months' practice.
If you find it difficult to visualise the whole picture try to visualise any part of the picture. Try to produce even a hazy picture. By repeated practice the hazy picture will assume a well-defined, clear-cut form. If you find this difficult, fix the mind on the effulgent Light in the heart and take this as the form of the Lord or Devi.
Do not bother yourself if you are not able to have perfect visualisation of the picture of the Lord with closed eyes. Continue your practice vigorously and regularly. You will succeed. What is wanted is Prema for the Lord. Cultivate this more and more. Let it flow unceasingly and spontaneously. This is more important than visualisation.