Although you can follow this guide without an SD card, you will not be able to create or restore a NAND backup using BootMii, and you won't be able to use some homebrew.
This guide is for the original Wii only, with the latest firmware (4.3), and the Wii mini. Do not use this on a Wii U (vWii)! If you want to install homebrew on a Wii U, [follow this guide](https://wiiu.hacks.guide).