Can someone link me to a detailed step-by-step tutorial(or video) on how to make this multiboot haxchi with quickstart method work? I'd really appreciate that.
Sorry, it's not a step by step, as it will require different hack and tools.
you probably want to see the readme on fix94's github :
https://github.com/FIX94/haxchi
and the official thread :
https://gbatemp.net/threads/contenthax-a-vulnerability-in-wii-u-file-system-verification.447688/
it works by using WUP server to replace a VC DS game with a homebrew. (quick explanation)
you need to officially buy a WiiU VC DS game on eshop from the list here :
https://github.com/FIX94/haxchi/releases
then use IOSUHax's fw.img (the one with WUP server only, not redNAND) to access your internal files and replace the "rom.zip" of the bought game with the one provided by Fix94.
Launching that VC game, it will auto-boot HBC from your SD card without launching the browser or needing an internet connection or local web server.
You can customize your VC DS game (edit it's icon, splash screen, etc.) and you can also add a "config.txt" redirecting to different homebrew or fw.img to load at launch based on the pressed button.
someone released a "haxchi" installer today :
https://gbatemp.net/threads/release-haxchi-installer-1-0-noob-friendly.448890/
not tested a lot yet, it's new. But you can do it manually instead with fw.img, wupserver and follow github's readme.
once installed, you can enable the wiiU quick start option in the settings to choose that game and boot directly into it instead of the system menu.
using the quick start menu is safer, don't mess with "coldboothax" (system.xml)