From the book "Jap Ji – The message of Guru Nanak", written by Sant Kirpal Singh

The three restraints are to be exercised by training the faculties to work inwards. The first restraint, that of speech, consists in doing Simran or the constant repetition of the name of the Lord with the tongue of thought. The agency of lips and tongue is not needed. The use and the effect of simran have been dwelt upon in the previous pages. The second restraint refers to the faculty of sight, and consists in contemplation of the spiritual patterns within, which open out in full effulgence. If you practise Tratak or concentration at some black spot outside, with your eyes fixed on it, your own inner light is projected outside. Accordingly, when you fix your inner gaze just midway between and behind the two eyebrows (the seat of the soul in man), you see your own light inside. That light is yours and is already there and you have to fix your inner gaze on it. Where there is the Word –  Holy Naam – there is Light; where there is Sound, there is Refulgence for the two are inseparable. The Light patterns are of five kinds corresponding to the five different kinds of Sounds, which are seen and heard as the spirit ascends upwards into the five different planes. The Monduk Upanishad speaks of these as the "five fires in the head."

Some scriptures prescribe the fixation of the inner gaze on the facial form of the Master, so as to make the mind stable. But you do not have to meditate on the flesh and bones of the Master's face, but on something shining forth through it. The face, especially the eyes and the forehead, constitute the seat for the full play of the spirit of the Master. Hence, to meditate upon the Master's eyes prepares a soul for the dawn of Divinity, by imbibing His Nature. This helps the devotee a good deal. As you think of the Master, you rise in the Master Himself. As you think, so you become.

The form of the Saint shines forth in full effulgence.
The sages and the seers live in the timeless One
and are fit to be meditated upon.
Sarang M.5

The Master's form is the nearest approach to Divinity, for He is the Son of God. One who doth not know the Son, cannot know the Father. Therefore, it is said:

Meditate on the Form of the Master.
Take His words as Gospel truth;
Let the footfalls of the Master resound in the recesses of your heart.
The Master is the Infinite, bow thou unto him.
Gond M.5

Think of the Feet of the Master in your heart;
By constant remembrance of Him,
you will cross the ocean of illusive matter.
Gond M.5

By meditating on the Form of the Master,
thou shall be honoured both here and hereafter.
Gond M.5

Again:
Remember the true Master within yourself,
and with the tongue repeat the Word given by the Master.
With the eyes visualise the Form of the true Master,
and with the ears hear the Holy Naam – the Eternal Song.
He who is saturated within and without in the true Master, the Word Personified,
gains a seat of honour in His presence.
Nanak saith, on whomsoever the Lord showers His grace, He bestows this state.
But few be the chosen ones, who obtain this gift.
Gujri War M.5

Here we have to be on our guard. If, however, the Master, on whose form you meditate, is not really spiritual, you will become what he is. This course, therefore, is dangerous, unless you are sure of the perfection of the Master. But you cannot differentiate the right person from the wrong. It is, therefore, safer to fix your inner gaze on the Light which a competent Master will give you at the time of initiation. The  true Master will gradually appear within, in His own radiant form, after some practice, the genuineness of which you can always verify by repetition of the charged words given by the Master. Only a competent Master can appear within at the time of initiation or later on in meditation after some practice. This will save you from any deception or temptation.

The third restraint relates to the faculty of audition. This is to be attuned to the Eternal Song reverberating in and through all. The Sound is the real essence of the Lord.

The Master says:
O my ears, hie on, hie on; hear the Truth
for which you were attached to the body;
And listen to the Eternal Music – the true Bani.
Ramkali M.3

It does not mean that we are to neglect the use of these organs in relation to the outer world. But these are to be so trained that they may become positive aids in the way of spiritual development, thus conveying a two-fold benefit. The training is meant to make the mind self-centred and prevent its stuff from needlessly flowing outside. The first process, Simran, forms the basis of spiritual pursuit. It has to be continued till the goal is reached. The second and the third, Dhyan and Bhajan, follow of themselves one after the other.

Restrain the mind stuff from ebbing away
through the nine portals in the body;
This will gain thee access to the tenth door
leading to the true Home of thy Father.
There the unending Music is resounding day and night.
Through Gurmat (instructions of a God-Man) this Song is made audible within.
Majh M.3

Wajhan Sahib saith:
How dost thou say that the husband is away?
For thou canst see Him by closing the ten doors;
Then wilt thou hear the Eternal Music beat on thy ears,
And thou shalt be transformed from a serf to a king.
All kinds of melodies are in the body.
And enchantingly sweet song is being set afloat.
O Wajhan! whosoever hears this song, great is his fortune.

These three restraints help in concentration to the highest degree. The first, Simran of the charged words given by the Master, brings back the mind from outside and withdraws the spirit from the body to the seat of the soul in the body. This is first sinking from the circumference of our being to the centre of our being. This also keeps reminding us of the highest ideal set up before man, which is self-knowledge and God-knowledge. The second process – Dhyan – also helps in concentration and fastens the soul inside. The last process – Bhajan or listening to the spiritual music in man – carries the soul into the beyond, to the source from whence the life current or sound principle of the Holy Naam or the Eternal Song, emanates. Just as in a pitch dark night when one sees naught or knows nothing before or behind, the bark of a distant dog or the far off flicker of a candle light helps one onward in his journey, so does the spiritual song and the refulgent light help a probationer soul on the Path in its solitary ascent to the true Home of the Father.

The long process begins with concentration at the seat of the soul in the body, situated just, between and behind the two eyebrows, where mental Simran is done in full earnest. This enables the withdrawal of spirit current, at present diffused in the body, and concentrates it at the seat of the soul, with the result that confines of the gross matter of the body and of the outward world are cut off. The spirit once unfettered and disimprisoned from the finite existence, now gets an ingress into the Til, Nukta Sweda or the Third Eye, and from here proceeds further, with the help of a Godman, to higher planes within. After passing confidently through the astral planes, one reaches the Dasam Dwar, with its sacred fount of nectar, the true Amritsar, Mansarover or Prag-Raj in man. The Muslims call it Hauz-i-Kausar. A bath or baptism therein frees the spirit from the sheaths of the astral and the causal bodies and finer matter. This is the real baptism with the holy-waters of immortality. The spirit now left to itself, is fully refulgent with a Light brighter than that of several suns. Now cognisant of the true essence which is none other than that of the Lord Himself, it proceeds further with the help of the Master in His Radiant Form until the spirit reaches the pure spiritual region: the Sach Khand, New Jerusalem or Muqam-i-Haq, where Sat Purush – the Formless One – resides. Therefrom with the help and through the grace of the latter, the Spirit is passed on to the Nameless One, stage by stage. This aspect of the ascent of the soul from material planes to spiritual-material regions and thence to purely spiritual regions, forms the subject matter of stanza 21 of the Jap Ji. Therein the Master has dealt with the three most important stages out of the five spiritual stages: Til (the starting point), Dasam Dwar, and Sach Khand. The five regions are also explained at the end of Jap Ji.

The man who has access into Til and bathes in the lustre of the luminous astral figure of the Master, is called a Sikh or a disciple of the Master (a devotee). As he progresses upwards, he reaches the Dasam Dwar, the third in the stages of development, and becomes a Sadh (a disciplined soul). When he reaches the region of pure spirit, he is given the epithet of Sant or Saint. One who has reached the highest spiritual region of the Nameless One, the Unknowable and the Imperceivable, becomes a Param Sant, the Saint of Saints. These expressions occur in the text of the sayings of the Masters. These are specific terms with significant connotations and have no reference whatsoever to those engaged in the outer pursuits of forms and formularies, or in the performance of rites and rituals or observance of fasts and vigils, etc.

Guru Nanak was a Saint of the highest order, the Param Sant who reached the region of the Nameless One, as will be evinced from His own words:

Fly above the region of Truth, of the pure spirit,
then reach the stage of the Unknowable and the Imperceivable.
Above this is the abode of the saints, and lowly Nanak dwells there.