Nintendo Network ID - How?

Have been searching a bit, but can't even find it explained at the Nintendo website. Checked every menu in my 3DS and its eShop, too - nothing

I would like to use my Nintendo Network ID from the WiiU eShop on the 3DS as well. That option has been integrated recently, but I can't figure out how to do it. Anyone here so kind as to help me with this?

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