Ignore your diaphragm and sing from your upper throat. This will feel unnatural to professionally trained singers, but it is key to the falsetto range. Singing this way causes the outer edges of your vocal cords to vibrate. You'll know when you're singing falsetto because you'll feel the strain in your throat. Always begin slowly and don't overdo it. To ensure you're singing falsetto, place your hand on your chest and feel for vibration. If you feel your chest vibrate, you aren't singing in the falsetto range. A vibrating chest means your diaphragm is still in play. Take a deep breath and start again, moving your voice out of your diaphragm and into your throat. You can use your breath to "lift" your voice.
Scales are essential for practicing and enhancing your falsetto. Since falsetto is the softest form of the human voice, you will never achieve the dynamics you can when you sing from your diaphragm, but you can extend the dynamic tone and range to help prevent squeaking. Sing scales regularly in falsetto, varying the rhythms and patterns of the notes you sing. Sing your scales ascending and descending. As you sing your scales, concentrate on breathing and using the air to lift your voice.
Remember that falsetto is always lighter than a normal singing voice. Never force your notes because you will lose control of them and make some very distracting sounds Falsetto is confined to your upper throat. You can feel the limitations when you sing falsetto. Any forced air will push you away from that range, causing you to sing off key. Try to visualize your voice going higher as you raise your voice to the falsetto range, keeping your breath gentle. The visualization technique is effective and helps overcome your natural instinct to force your breath out.