This is something I used to practice on a regular basis, when I was in US Army deep sea dive training. Always practice new techniques ABOVE water till you understand all the variables innvolved. First, if you haven’t already, try filling your lungs with as much air as possible. While holding your breath suck more air into your mouth and force it into your lungs as well. In time, you will expand your lung capacity but, you have to pace yourself or you’ll injure yourself. Once your lungs are filled to capacity, in your mind, sing a song to yourself. Make it a song that you know word for word, (I chose Beastie Boys’, Paul Revere) and try to make it to the end of the song before letting your breath out. Now, speed it up annd try to sing it twice. Remember to remain totally relaxed (I often layed in bed without moving) so you don’t waste energy or concentration. When you attempt this in water; you will have more air intake capabilities and your body will start to operate better as your oxygen saturation begins to drop. Now, sing that song while swimming uderwater. How far do you get? Document and repeat; this is more than practice, its training.