At this the Mantra-Mediation has the special purpose, to create a vibration which strengthens itself by the constant concentrated repetition , which transforms those regions, which come into contact with this vibration. The deeper the meditation the bigger are the effects of such a vibration.
Prerequisite for such meditation is traditionally the control of the thoughts. This has the simple grounds in the fact, that thoughts disturb the concentration on the Mantram. Otherwise such concentration is a cramped holding tight at the mantram. It should be an "IT - istic" looking recur, corresponding with the state of consciousness of the student. : IT acts in the transcendent areas....

A futher reason of the necessity of the previous control of the thoughts is simply, that storms of thoughts that happen in deep meditation can have later chaotic effects, when the deep mental waves come into realization in life. As soon as such caotic thoughts have transcendental effects in the akasha-principle, they can have strong karmic effects. Therefore a deep meditation with storms of thoughts can have very negative effects, even if the student notices that and then returns to the mantram, as it is handled in the "Transcendental Meditation".

Another point is, that the deep meditation about a mental content or a mental theme does not take into account, that mental waves have a quabbalistic aspect, so that such thoughts can often have very negative unconscious effects but must not have . (--> Fr. Bardon : "Key to the True Quabbalah")

Generally meditation has three steps :

1. Preparation : Attainment of a quietness, for example through the concentration on the breath or using Mantrams, which have such effects(in Taoism "S-Ung" (quietness) or "F-ANG" (relaxation) , or even through observation and rejection of upcoming thoughts. Another method is to read a chapter of a holy book, so that the contents of the momentary thoughts are tuned.
2. The Mediation with a certain technique within several steps (until the later Samadhi)
3. The return to the normal consciousness

Other Methods mainly use techniqs with forms and colours :

Here we can find methods of meditation over elementary forms like triangles and Mandalas, of pictures of deities or buddhas or masters, over cosmic mandalas with a great variety of forms and colours.
Such visualisations make use of the lawful contents of colour and its meaning, and of form and force and of the qualities of those forces. (the Feng Shui offers to this one disputed explanation).
Especially in Tantrism (Mandala, Yantra) and even in Buddhism we can often find such visual meditations. Generally these techniqs were used by progressed students with trained abilities of visualization and imagination.

The simultaneous use of sound, colour and meaning makes these exercises much more effective, but because they are difficult to practice, the practise of the single aspects will mostly be necessary before that.
Therefore Franz Bardon teaches the use of letters in combination with colour and specific feelings in his book about the Quabbala, and he explains their individual meaning. For example in hinduism the meaning and the colours and characteristics of a deities were exactly explained by a real guru before initiation.