On the Main Menu tap the “Voicemails” tab. Then tap the small microphone at the top right. On the next screen, click “Custom”, then tap “Record”.
You can listen to your recording, and re-record as much as you like.
When you’re happy with what simply select your custom voicemail as the default and tap the check mark to save.
NB. Make sure you allow access to your device’s microphone, or the app won’t be able to record you!