Assuming you would like instructions on how to record a video using the built-in camera app on an iPhone:
Open the Camera App.
Select Video recording.
A red Record button will appear at the bottom of the screen, along with a timer that counts up while you're recording. To stop recording, press this button.
You can switch between the front and rear cameras by tapping the icon in the top-right corner of the screen.
To change other settings before recording, tap . These include turning on Live Photos (which records a short movie along with your photo), choosing whether to use filters, setting a timer for taking your video, and more.