FORMLESS MEDIATION:

If none of the Deities [Different Forms of God] appeals to you, then you  can meditate on the Supreme Being as the light within your being and repeat 'Om' which is the Universal name of God. Please you follow other common procedures.

Sri Ramakrishna explains about Formless Meditation: 

SRI RAMAKRISHNA

“There are two kinds of meditation, one on the formless God and the other on God with form. But meditation on the formless God is extremely difficult. In that meditation you must wipe out all that you see or hear. You contemplate only on the nature of your inner self. While practicing this form of meditation one directs one’s look to the forehead. It is meditation on the nature of one’s one inner self after negating the world, following the Vedantic method of Neti, Neti- Not I, Not I. When I meditated during my Sadhana, I used to think of the unflickering flame of a lamp set in a windless place.

         *                      *                   *                   *                   *

OTHER TYPES OF FORMLESS MEDITATIONS:
The aspirant can imagine himself or herself as a –
1.Fish swimming in one indivisible ocean of Existence- Knowledge- Bliss:
INTERESTING INCIDENTS: 
i.Visiting Sri Ramkrishna, oneday a Brahma Devotee asked, "Sir, how do you feel in samadhi?" He replied, "I feel like a fish released from a pot into water of the Ganga." 
[Source: The Gospel of Sri Ramakrishna]

ii.Once Sri Ramakrishna took Mahendra Nath Gupta [M.], the author of 'the Gospel of Sri Ramakrisha' to a lake to teach him how to meditate on the formless Brahman. Fearlessly fish swam  in the water and joyfully sported there as nobody  was allowed to catch them. He told M.: "Look at the fish. Meditating on the formless God is like swimming joyfully like these fish in the ocean of Bliss and Consciousness."
[Source: The Gospel of Sri Ramakrishna]

2. Pitcher fully immersed in water- water inside and outside:
SRI RAMAKRISHNA: "Sometimes I considered my self to be a pitcher immersed in the water of that indivisible Satchidhanada which pervaded me through and through." 
Source: Sri Ramakrishna and his Divine Play

3. Bird flying in the infinite sky:
"Every soul is a young eagle soaring higher ad higher, gathering more and more strength, till it reaches the Glorious Sun (God) [Source: Complete Works of Swami Vivekananda]

4. Point of self- conscious light immersed in one indivisible ocean of Light.

a) Sri Ramakrishna's  Spiritual Experience: 
Once Sri Ramakrishna visited Devendranath Tagore, the father of Rabindranath Tagore. Tagore said him, " This universe is like a chandelier and each living being is a light in it. Sri Ramakrishna also told his spiritual experience: "Once meditating in the Panchavati, I too had a vision like that."
SOURCE: "The Gospel of Sri Ramakrishna"

b) Swami Vivekananda in Practiacl Spirituality told about individual self:
"Man is an infinite circle whose circumference is nowhere, but the centre is located in one spot; and God is an infinite circle whose circumference is nowhere, but whose centre is everywhereIt is important that the aspirant has to take formless meditation by following the below steps. Sri Ramakrishna used to insist that a shooter should aim at a fixed target before attempting to shoot a flying object. [ C.W  of Swami Vivekanada]

1. With form and with attributes;
2. Without form and with attributes;
3. Without form and without attributes