Canon Speedlite EL-5 Advanced User's Manual page 131

2.
Set the flash mode to <
Press the joystick vertically or horizontally or turn <
< >, then push the joystick straight in.
The receiver flash mode is set automatically during shooting, as
controlled by the sender.
3.
Set the firing groups of the receivers.
Set a firing group (A–E) for each receiver.
>.
131
> to select