Open the Messages app
Open or start the conversation
Press and hold the microphone icon next to the text field
Speak while holding the microphone
Release the microphone to send
To cancel, slide your finger left while holding the microphone
If you prefer before sending, swipe up on the microphone to lock recording, then tap send when finished
For FaceTime, open a call and use the microphone to record/send a voice message if your iOS version supports it