If I need to simply explain, let us do kapalbhati in this way:
First sit on the ground either on vajarasana or sukhasana (no need to sit in tight padmasana, any comfortable asana can benefit if pranayama is done correctly). Be on empty stomach.
Second step is to exhale forcefully through nose but using the muscles of the abdomen (navel portion) (do not do if you have spinal or back pain in lower lumber region or have recent surgery in this region); do not bother about inhalation. Increase frequency of forceful exhalation slowly upto 30 strokes per minute. There will be automatic inhalation with exhalation. So, this is called active exhalation and passive inhalation. This is one round of 30 strokes. Start with 5 rounds of kapalbhati initally. Also, you can increase the strokes per round from 30 to 60 or even higher by practicing for weeks.
I hope that this is enough for you in the beginning.
Keep the spine straight while practising the kapalbhati pranayama, especially from lower region.
Feel free to write again for more explanation.
