Depending on your iPhone model:
On an iPhone 8 or later (including the original SE and SE 2020):
- Press and quickly release VOLUME UP
- Press and quickly release VOLUME DOWN
- Hold the SIDE button until an Apple logo appears (15-30 seconds), ignoring "slide to power off"
The operative word here is QUICKLY; and try it a couple of times, the timing is a little tricky.
- On an iPhone 7 or iPhone 7 Plus: Press and hold both the Side and Volume Down buttons for at least 10 seconds, until you see the Apple logo.
- On an iPhone 6s and earlier, iPad, or iPod touch: Press and hold both the Home and the Top (or Side) buttons for at least 10 seconds, until you see the Apple logo.