This was a 'breathing trick' I learned from one of my 'voice teachers' many years ago! This 'exercise' will 'teach you' the 'correct way' to do "diaphragmatic breathing."
I want to stress right off that this is not the only one but in my opinion the best one to start with to get the idea.
OK, First find a nice big fat telephone book, or something equal in weight like a heavy 'book.'
Lay down on the floor and place the 'telephone book' right below the bottom of your rib cage in a 'horizontal position' across your stomach.
Now take a 'deep breath' in and as you do watch how the 'telephone book' "raises up!"
Now let the air out quickly and watch how the 'telephone book' drops down!
You can try taking a 'deep breath' in first through your 'nose' and then letting it out through your mouth and then through your mouth and again letting it out through your mouth, either way.
After getting the 'idea' of how to do it and seeing how the 'telephone book' goes 'up and down' with each inhale and exhale you will take it a step further.
Now, take in a 'deep breath' through your mouth or nose but this time I want you to "Hold the air in and 'control it' by blowing it out in a very 'slow and steady, hissing sound," like letting the air out of a balloon.
This 'exercise' will 'teach' you not only to learn how to do 'diaphragmatic breathing' but also how to 'control' your 'breath' so that you don't run out of 'air' before the end of a 'phrase' unless you want to for your own 'creative technique.'
I warn you this 'exercise' can be very 'boring' and probably 'hurt' a bit until your 'muscles' get strengthened up just like any other exercise. But you have to keep on doing it until you can do it naturally without effort 'standing up!'
"Good luck," and much success in your endeavor.