Self-Timer Mode - Nikon D3400 Reference Manual

Self-Timer Mode

The self-timer can be used for self-portraits or group shots that
include the photographer. Before proceeding, mount the
camera on a tripod or place it on a stable, level surface.
1
Press the I (E/#) button.
2
Select E (Self-timer) mode.
Highlight E (Self-timer) and press J.
3
Frame the photograph.
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I (E/#) button