This is a simple affair but not easy. My first experiences with the double I experienced on psychedelics. Then i was granted it whilst sober. Since I have had over a long period of time many experiences in this world and in dreaming. I say it is simple because if humans have a goal and commits to it they attain that goal. I say not easy, because you need silence, discipline, patience, focus and unbending intent, to name a few qualities. Also you need correct information and technique.
Tantra tells us that the prime quality for attaining siddhis (powers) is purity. Many preliminary techniques of tantra and yoga are for bringing the body to purity.
However you attain trance, this is the state you desire to access these higher talents of the body. Do Sorcery Passes or Tantric Yoga or another practice that appeals to bring you to peak health and vitality. Do Pranayama or energy practices to slow down the thoughts, elevate the mind and make the body more sturdy. Give yourself to silence practice.
Be pragmatic, feel and think your way through and take care. This undertaking is not for the faint hearted or the self-involved.