Two interesting features of the latest 3DS update are Miiverse and the Nintendo Network ID.
The Nintendo Network ID feature allows users to share balances between a Nintendo Wii U and a Nintendo 3DS / 3DS XL / 2DS. In addition, it gives you access to the Miiverse service.
Nintendo made an excellent explanation on how to set your Nintendo Network ID (that means linking an existing one or creating it).
See the video below with nice music, easy explanation and a clean design. It’s perfect for the initial run of the system update: