Install custom firmware with Unlaunch
Install TWiLight Menu++
Use nds-bootstrap for running homebrew and backups
Add a compatible SD card adapter or flashcart
Format the SD card to FAT32
Copy homebrew apps to the SD card
Use a compatible exploit or already-modded system to launch the installer
Backup NAND and system files before making changes
Install HiyaCFW for SD-based system menu booting
Update Unlaunch and TWiLight Menu++ when needed
Keep a copy of recovery tools on the SD card
Avoid modifying system files without backups
Use only DSi-compatible homebrew and software