Step 1: Connect your iOS device to the Mac using the Lightning cable. You may need be asked on your iPad to Trust this computer.
Step 2: Open QuickTime Player, which can be found in the Applications folder, through Launchpad, and a number of other ways. A dialog box to open a file will usually appear, which can be dismissed by clicking Done without selecting a file.
Step 3: Click File in the Menu followed by New Movie Recording.
Step 4: Click the arrow next to the record button, and select iPhone or iPad under Camera. QuickTime Player will resize and change the image to mirror the iOS device's screen.
Step 5: Use the same menu to configure the microphone. You can select the device again to capture audio from the iPhone or iPad, or if you want to record a live narration, select a connected microphone instead.
You do need to click the record button.