In some of his books, Samael Aun Weor has written about an interesting practice that deals with activation of chakras and their corresponding faculties by stimulating the chakras with solar energy. This is done by pulling up that energy from the solar plexus chakra and leading it towards the chakra that one wishes to activate. To achieve this, one uses visualization and particular series of mantras. Samael says the following in his book the Perfect Matrimony:

“The Kundalini or igneous serpent of our magical powers has a great deposit of solar energy in the umbilical region within the solar plexus chakra. This magnetic center is very important in initiation because it is the one that receives the primary energy which is subdivided into ten splendorous radiations. This primary energy circulates through the secondary nervous canals, animating and nourishing all the chakras. The solar plexus is governed by the Sun. If the student wants to have really vigorous objective clairvoyance in the most complete sense of the word, the student must learn to take the solar energy from its deposit in the solar plexus to the frontal chakra.

“The mantra SUI-RA is the key that permits us to extract solar energy from the plexus of the Sun in order to carry it to the frontal chakra. Vocalize in this way: SUIII RAAA. When practiced for one hour daily, the result will be the positive awakening of the frontal chakra. If we want solar strength for the laryngeal chakra, we must vocalize the mantra SUE-RA in this way: SUEEE RAAA. If we need solar energy for the lotus of the heart, we must vocalize the mantra SUO-RA in this way: SUOOO RAAA. Everything is summarized in the great SUA-RA, where according to the Vedas and the Sastras the Silent Gandharva (heavenly musician) is found.”
~ Samael Aun Weor

It is already mentioned in many of his books about the effect that the vowel sounds produce upon various chakras of the astral body. It is said that vowel “I” activates the third eye chakra, vowel “E” the throat chakra, vowel “O” the hearth chakra, vowel “U” the solar plexus chakras, vowel “A” the two chakras of the lungs, and sounds “M” and “S” are related with the navel chakra and the root chakra.

In the practice of directing the solar energy, the same correspondence of the mantra sounds with the chakras apply, but instead of just vocalizing the mantra sounds, one directs the solar energy with an aid of concentration, visualization, and vocalization.

The mantras for moving the solar force from the solar plexus chakras to other chakras are:

SUI-RA
SUE-RA
SUO-RA
SU-RA
SUA-RA

The practice can be done by using just one mantra for the entire length of the practice, in which case you would work only on one chakra. This is particularly useful if you need access to certain psychic faculty that the chakra you wish to awaken is related to. Like for example if you need access to intuition, you would work on the mantra that emphasize vowel “O”, which in this case would be mantra SUO-RA.
Another way that this practice can be done is by using two or more of the mantras, all depending on what your goal is with the practice. For example, if you wish to nourish the chakras with solar energy, you can do all five mantras during one practice. If your goal is to awaken only certain chakras, you can do mantras only for those chakras. Or, as already mentioned, you can do only one mantra and focus on one chakra throughout the length of the practice.

“The Student should note the accentuation of the vowels which form the fourth column, and the accentuation of the A in each Mantra.
According to the Vedas, the silent Gandana, the Celestial Music, is contained in the sublime SUARA.
With these Mantras of the Fourth Series, the Fire of the Solar Plexus is conducted to each of the Chakras of the Astral Body.
We insist that the first Mantric syllables of this series: SUI, SUE, SUO, SUU, SUA, are vocalized with the intonation of a diphthong accentuated on the last vowel, which will be prolonged for a long time.”
~ Samael Aun Weor, Logos Mantram Theurgy

If you choose to work with mantra SUI-RA for the entire length of the practice, then you can do the following: sit in a regular posture for meditation, close your eye, and relax the body of any physical, mental and emotional tension. Once that’s done, focus on the solar plexus chakra, seeing it as a glowing sun, filled with solar energy, which is the energy of life and of creation. Then inhale deeply, and upon exhalation vocalize the first syllable of the mantra SUI-RA, like this:

SSSSSUUUUUUUIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIII

“S” is briefly elongated, as well as “U” (pronounced as in word “loom”). While elongating “U” focus is on the solar plexus chakra, and then when vocalizing long “I”( pronounced as in word “liver”) you visualize the energy moving up from the solar plexus into the third eye chakra, awakening it, seeing it full of life and activity. Then take another deep breath and when exhaling pronounce the second syllable of the mantra, like this:

RRRRRAAAAAAAAAAAAAAAAAAAAAAA

The emphasis on this second syllable is on vowel A (pronounced as in word “after”), elongating it.
Syllable “RA” reminds of the ancient Egyptian god of the sun, which esoterically is associated with the Logos, the Cosmic Christ, the spiritual aspect of the sun. And the mantra SUIRA reminds of the word “Surya”, which means “sun” in Sanskrit.

When it comes to the length of this practice, it can be done for as long as it is suitable for you, or for as long as you feel you need to do it. You can start with 10 minutes, and then gradually increasing it to half an hour practice (or even longer if you wish). Doing this mantra for half an hour can be very beneficial, because if done correctly one can experience the spiritual energy of the mantra, of the stimulated chakras, as well as the energy arising from the connection with that spiritual aspect of the sun.