The 'Q' screen on the Canon DSLR for back button focusing

Press the Menu button on the left-hand side of the camera.

The Menu screen on Canon DSLRs for back button focusing

Move along to custom functions. This has a red symbol of a camera and five lines underneath it.

The Custom Functions screen on Canon DSLRs for back button focusing

Go to C.FnIV: Operation/Others and select option 1. This is indicated by the star in the bottom left-hand corner.

The Operation/Others menu on Canon DSLRs for back button focusing

A new menu custom control will pop up. Move to the AF-ON button. Enter the next step by pressing ‘set’.

The Custom Controls screen on Canon DSLRs for back button focusing

Select Metering and AF start, which is the first icon.

Assigning the AF-ON button for back button focusing

Press ‘set’ to assign the button as Autofocus. It will bring you back to the Custom Control menu. Press menu twice to go back to the original screen.

Showing the changed AF-ON button on Canon DSLRs for back button focusing

Other Canon cameras work as follows:

EOS Rebel T3/ T3i/T4i: C.Fn 7 (option 1 or 3)
EOS 50D/60D: C.Fn IV-1 (option 2 or 3)
EOS-1D Mark IV: C.Fn IV-1 (option 2 or 3)
EOS 5D Mark II/Mark III/ EOS 1-D Mark IV: C.Fn IV-1 (option 2 or 3)
EOS-1D X: C.Fn menu screen 5 (Custom Controls — Shutter, AF-ON, AEL buttons)

Nikon