Just press the red button to start a new recording.
Other than that, there are some other convenience ways to start a new recording:
1.Quick action on the app icon
If your iPhone (6s, 7, 8) supports 3D touch, deep press on the app icon, then quick actions will be shown, select “Record” to start a new recording quickly.
Other information maybe you need to know:
Incoming call or alarm clock would interrupt the recording. But the recording would be resumed automatically when the incoming call or alarm clock ends.
The audio hardware couldn’t carry different Apps at the same time. Therefore, please don’t open other Apps when you are recording. If there are sounds from other Apps, the recording would stop and will not resume automatically.
Reference: