Pixelchat is a drop-in replacement for Miiverse, which serves as a web-based Pictochat.
Note: Please use at your own risk.
Extract the contents on the zip to the root of your SD Card. The zip contains 2 folders, 3ds and luma.
Extract the folders in the same place you see them.
If you're asked if you want to overwrite the contents of the folders, please allow it to continue. It won't overwrite any file unless you're updating a previous patch install.
Insert the SD Card onto your 3DS if you haven't done yet, then press and hold the Select button while you power on your 3DS to access your Luma3DS configuration.
Once there, make sure you activate the Enable Game Patching setting and press the Start button to save.
You're done. The only thing left to do is to open the Miiverse icon on your Home Menu and you'll see Pixelchat.
The button Enter Rverse will redirect you to Rverse, which is a Miiverse clone formerly known as foxverse and Reverse.