BootMii is a modification of boot2 (which loads the system menu when booting the Wii). As boot2 is normally not updated through a wii system update (which was proven wrong with the 4.2 update!), this is the best brick protection you can get. Unfortunately, Nintendo already knew about it before it was even released, so they updated boot1 (which loads boot2) on newer Wii's. This is why Bootmii can't be installed as Boot2 on those Wii's.

Using bootmii is not difficult. Every time you boot the wii with the prepared SD card inserted, you will see the Bootmii menu. You can navigate through the menu with a GC controller or the buttons on the wii (Power = right, Reset = OK).


The bootmii menu                                                                       

To make your wii brick-proof, make a backup of the NAND before making major changes to the Wii memory. The NAND backup function can be found under the 4th button, then the 1st button (the highlighted button in the below screenshot).

The first option is NAND backup, the second is NAND restore     

If you get any Bad blocks during the Backup, don't panic! This is completely normal. Once it completes making the backup and verifying it, put your SD card in the computer and Copy the NAND.bin to a safe place.
Also copy over the keys.bin file. These are the unique keys of your Wii!

If you need to restore your nand backup, place it (the nand.bin) back in the root of the SD card and choose the restore NAND option (to the right of the backup NAND button).