Breathe from your diaphragm, using a technique known as diaphragmatic breathing. This involves taking control of the normally involuntary diaphragm muscle and using it to take in and hold air.
Push air out of your lungs using the diaphragm. This should not be confused with belting, but should be a slow controlled function that allows a consistent and powerful stream of air.
Start on a pitch an octave below the top of your vocal range and sing a major scale. As you switch from chest voice to falsetto you will notice a physical change between the more dense and musical chest voice and the thinner and pure falsetto.
Practice singing between chest voice and falsetto to develop the muscle memory required to switch easily between the two voice registers.
Sing five-note scales in the falsetto range. Take a break after each scale to avoid tiring your voice. These five-note scales should ascend and descend. Use a major scale as your guide for the first five pitches.