if you wanna hold your breath for a long time dont force it (none of this is advice or telling you to do it) but if you do happen to be near a trained and certified freediving professional they would probably tell you to relax your body take a deep breath not in a way that strains or tenses your muscles tho and then just sit there around 1 minute in, it will become extremely uncomfortable but if you fight through that you'll get a strong second wind another thing is that you can trigger the mammalian dive response with enough training