First, learn Nabhi Kriya in the videos below. In the first video, you will learn Nabhi from a visual perspective. In the second video, you will learn Nabhi from a feeling perspective (watch from 24.46.) Then, learn the basics of Kriya Pranayam in the third video (watch at least from 29:40-58:00.)
This is the main (Spinal) Kriya Pranayam Yoga technique (third video), described in words :
After you have intuitively “felt” the Chakras within the Spine and you are aware of their position within the Spine, the next thing is to move the breath up and down the Spine and, with your breath, touch each of the Chakras softly and gently as you move the breath upwards and downwards.
You begin the practice by first inhaling the breath upwards from the 1st Chakra (the Mooladhar Chakra at the base of the Spine) and chanting OM there. And as you continue the inhale upwards, you keep moving the breath up as the breath touches the 2nd Chakra (Svadisthan), then the 3rd Chakra (Manipura), then the 4th Chakra (Anahata), then the 5th Chakra (Vishudhha) and finally the 6th Chakra (Ajna Chakra) mentally chanting OMs at each Chakra as you inhale fully and deeply (without straining yourself.)
Once you reach the Ajna (the 6th Chakra at the medulla) there will be a slight, natural pause, as you begin to exhale the breath downwards from the 6th Chakra to the 5th to the 4th to the 3rd to the 2nd and finally to the 1st Chakra (at the base of the Spine) mentally chanting OMs at each Chakra on the way down. In order to locate the medulla at the top of the spine, raise the chin a bit and tense the muscles at the neck where one can feel the bony structure. That is the medulla.