Back button focusing involves decoupling a camera’s autofocus from the shutter button and moving it to a button under the right thumb on the back. Here’s a helpful 4.5-minute video by Canon Australia showing how you can get started with this next-level focusing technique on your Canon DSLR or mirrorless camera.
Canon Collective Ambassador Kass Brumley starts out by explaining what back button focus is and what its advantages are (e.g. being able to easily lock focus when needed through multiple shots), and then she walks through the steps involved in getting it set up.
The four basic steps are disabling autofocus from the shutter button (1:53), selecting “Meter Start” (2:13), assigning your AF-ON button (2:33), and selecting the focusing mode (2:53).