Simply go to System Settings > Other Settings > System Update. Once the update’s finished, the Miiverse software will be present on your home menu in the top right corner. You can create a new Nintendo Network ID, or link an existing one to get started. The above video from Nintendo shows a step-by-step process of how to do this quick and easy.
Once you’ve entered all account information, you can proceed to use Miiverse to post your thoughts in certain 3DS title communities. Note: 3DS Miiverse users cannot receive/accept friends requests or send personal messages at this time. While, if you’re linking your Wii U NNID, make sure all information you enter is correct, as you’re only able to have one ID per 3DS system. If you have the ID on both your Wii U and 3DS, all Nintendo eShop funds and purchases will be linked between the accounts.
Have you had a chance to update your 3DS system yet? If so, what do you think of Miiverse?