How do you use your diaphragm?

Put it this way. When you scream or yell loud, put your hand on your stomach. That is the way you use the diaphragm. You can yell loud or soft and this is controlled by the diaphragm. Singing is the same. the best way is to lie on the floor, put your hand on your stomach and breath in and out. the hand will rise and all. Sing in that position and you will get the hang of it immediately. Stand up, breathe in and start singing softly. Not increase the volume and you will notice the stomach tightens up from the inside, not the surface. Don't force. Be natural and you will feel the muscles tightening (diaphragm).It's not suppose to hurt. The only place it hurts is when you are doing it wrong and you get a sore throat. When you sing from the diaphragm, you can sing for hours without any pain whatsoever. When it hurts, it's because you are doing it wrong.

What feels tight? Your throat? If so, forget diaphragm until that's sorted out. It's very easy to learn how to blow hard using the diaphragm Doing so through a tight throat can do real damage. You need a singing teacher.