First, a deep friendship with darkness is needed.

During the Dark Retreat we should do some meditation with open eyes and others with closed eyes. We should constantly keep the Witness Consciousness, asking the question ‘Who am I?’. Here it is even more important (than in regular life) not to dramatize or let ourselves be taken over by imagery. In this way, we develop the capacity of witnessing any thought, sensation or emotion that may appear. The attention is on the darkness as the expression of the absolute. Exactly as the silence that appears at the end of the sound of a bell is revealing the ineffable dimension of our being, the feeling of darkness reminds us constantly of the transcendent Self. Traditionally the dark retreats were done by advanced practitioners and the period varied from a few hours to decades. Longer periods require stability in the natural state.
An extraordinary and very important spiritual phenomenon related with the meditation in darkness for long periods is the dissolution of the identification with the physical plane. In the absence of strong sensorial inputs that could bring the body to consciousness we have no reason to suppose that we are just this physical body. Because we cannot see our hands, legs, trunk etc, we realize that the pure presence, the feeling of ‘I AM’ is equally vivid in the physical body and around it, to the left, right, top, bottom, front, or back… Thus, the feeling of freedom from identification with the physical body appears naturally and i obvious. We have no reason anymore to affirm that we are this body, since the entire space is revealed to us as the ‘new body’, in which our consciousness can manifest freely.

It is recommended not to adhere to a fixed schedule, but to develop an intuitive attitude for phases of practices (meditation, Hatha Yoga etc) and rest. The practice of Hatha Yoga is recommended, but in a slow and meditative way avoiding any forceful attitude.
The awareness will increase and there is no need to be afraid of boredom. Here we will realize that boredom is a very relative concept, because it is related with the desire and hunger for stimulation. The deep relaxation and awareness gradually eliminates this constant need for stimulation.

“When some sense organ is obstructed in its function by some external cause or in the natural cause or by self-imposed device, the aspirant becomes introverted, his mind is absorbed in a void that transcends all duality and its essential Self is revealed.”
– 89th Sutra of Vijnana Bhairava Tantra