The Sons of the Pioneers sing Bob Nolan's "Way Out Here". It's an excellent way to start because it's not too busy and requires a relaxed sustain falsetto. It starts out on a high tonic note, dips down a half step, back up again and then down a scale pattern which
drops down a fifth into the chest voice, back up a major sixth, repeats that and winds up on the fifth. Then it starts high on the third of the dominant seventh, down a half step, back up and down a scale pattern then down a sixth to the chest voice, back up, down and back up again and winds up on the third of the tonic chord.

Be careful!   Relax the voice and work for a smoothness, not a harsh break between head falsetto and chest voice. You can hurt your voice if it's done too forcefully.

If you do "Wimoweh" be very careful not to go abruptly into the falsetto from a "shout".

Frank