Nikon D3400 Triple Lens Parent's Kit Owner's Manual

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66 More on Photography
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.
Self-Timer Mode
I (E/#) button