I have just tried the 25 times cyclic hyperventilation ventilation, i felt slight anxious but not that much, it actually feel good, its kind like smoke a cigarette, I have done that a few times, How to hold the breath after the exercise, meaning hold it right after the last exhale or inhale again and hold?

After completing a session of cyclic hyperventilation, the recommended approach is to exhale fully and then hold your breath with your lungs empty. This practice is emphasized particularly in methods like the Wim Hof technique, where it's common to breathe deeply then exhale fully before holding your breath for as long as comfortable, which is usually between 15 to 60 seconds. This breath-hold phase is intended to allow you to explore maintaining calm despite the heightened arousal caused by the previous hyperventilation 1 2.

is advised against performing these breath holds near water to avoid risks like shallow water blackout, as hyperventilation can significantly shift the threshold for the gas reflex by lowering carbon dioxide levels 3.