I've tried all techniques but they don't work. I'm a beginner so please don't be sharp.
Technically, you can't.
The diaphragm controls your breathing.
Waht you have to do is take very deep breaths, and make sure your rib-cage expands slightly when you do.
The diaphragm is a muscle which pulls down to make room for air to enter.
A good way of finding out where the diaphragm is, is to clench your buttocks together.
You will feel it pulling down.
I conduct a choir, and we practice breathing.
We breathe in to a count of four, hold for four, and hiss for four.
Then eight, then ten, then twelve etc.etc.