Canon EOS Rebel T5i 18-55mm IS STM Kit Instruction Manual page 232

Easy Wireless Flash Shooting
Setting [E-TTL II meter.] to [Evaluative] is recommended.
Even though the firing of the built-in flash is disabled when
[EasyWireless] is set, it will still fire a small flash to control the slave unit.
Depending on shooting conditions, the flash fired to control the slave unit
may appear in the picture.
Firing a test flash is not possible with the slave unit.
232
Select [Built-in flash settings].
4
Select [Built-in flash settings], then
press <0>.
Select [EasyWireless].
5
Under [Built-in flash], select
[EasyWireless], then press <0>.
Set [Channel].
6
Set the transmission channel (1-4) to
the same one as the slave unit.
Take the picture.
7
Set the camera and take picture in
the same way as with normal
shooting.
Exit the wireless flash shooting.
8
For [Built-in flash], select
[NormalFiring].