Imagine a beautiful swan feather. Put it in your palm, bring it to the level where your mouth is. Blow toward it gently so it leaves your hand and floats in the air, until it lands on the grass.
Remember what you felt in your chest and abdomen. As you ready yourself to blow, did you feel the breath fill up the chest? Did you feel your mid-abdomen tighten quickly? Remember, this is an association exercise using an object of imagination such as a feather in order to teach your body the right movement.
Do this exercise several times. Imagine blowing the feather up, far, or sustain for several seconds. With each attempt respectively, you will be practicing singing high notes, long notes or sustained notes.
In reality, much less poetic but more practical version will be, doing this exercise using a tissue paper attached to a stick. It's very simple, but an effective exercise. Good luck!